The problem with Heal is that it doesn't scale well once you get to the higher levels. It's designed to do healing based on a level 18 base HP bar, but that's not taking into consideration any items in the post-18 game. Plus it's countered by reduced healing effects like Ignite, Katarina's Death Lotus, Miss Fortune's Impure Shots, etc. While all heals are affected as such, in this case it's a tad more game breaking since it's eating up one of your summoner slots.
Revive is generally a poor choice due to the fact that it requires you to be dead in order to use it. Obviously you want to avoid being dead in the first place. While you generally can't avoid at least a handful of deaths in a game, you don't want to count on Revive being used as quick-back from death, compared to another skill which could have prevented that death in the first place.
That all being said, there are a few champions that can utilize these skills particularly well. A lot of support characters can utilize Heal quite well like Taric, Soraka, Sona, Alistar. Combining Heal with their built-in heal spells can provide an inordinate amount of burst healing which can be deceptive to the enemy team.
Revive is a good gimmick choice for a Twisted Fate or Pantheon playing mid to push the lane early. Once you're 6, go for a skirmish with your 1v1 opponent. If you win, great! But if you die, no big deal. Immediately revive, buy (be ready!) and ultimate back in, using your Gold Card (TF) / Shield Bash (Panth) to stun and finish them off. You have the speed boost from Revive so they wont escape, and now you have a good 15-20 seconds to push the tower. If you're well organized, you can have your jungler come help, or a sidelane move off to come finish the tower if you cant, keeping a strong 2v1 in another lane.
Additionally, Revive can be extremely strong late game when respawns are 50+ seconds long. You can turn a potential tower / inhibitor kill into nothing after a bad teamfight that's left you with few to no allies left. However, usually you don't want to ASSUME this is the fact, because you could have avoided that situation with a different summoner skill in the first place.
Basically, they're situational abilities with their place, but generally they're overshadowed by some of the more popular abilities.
I don't consider Nasus a true tank - he has no real Tank abilities, and serves better as a Tanky DPS / Anti-carry. That said, here's a list of the heroes I consider true tanks in the current metagame.
Alistar
I'll grant you that Alistar's hard to play optimally, but when he is, the bull's a force to be reckoned with. Two hard CCs (neither affected by Merc Treads). Triumphant Roar makes him an excellent pusher, and his Ultimate and high base health make you an excellent damage soak. Additionally, little of his power comes from items; Pulverize + Headbutt is really good even without any bonus from AP.
Amumu
Amumu's got two major things going for him: He's a great initiator, and he's a tank who can jungle and gank effectively. Due to the experience bonus, most top teams have a jungler on Summoner's Rift, and Amumu's one of the better ones. He also has two hard CCs in bandage toss and his ult, which can make escaping from him quite difficult (not to mention keeping folks within Despair for longer)
Cho'gath
Poor Cho just get nerfed pretty bad in the Nocturne patch, losing substantial damage numbers from feast. That aside, he still is a decent (if not a first pick) tank with a skill shot initiation ability (though Rupture is more easily dodge-able than some other initiators) and an AoE silence. There's also something to be said for the psychological and mechanical effects of having the largest model on the screen. You'll be shielding your allies from attacks simply because your opponents can't find them to click under a fully-feasted Cho'gath. After you've maxed your feast stacks, try using Feast earlier in fights rather than later: it does true damage, and it's a pretty sizeable nuke - carries especially tend to panic when 1/4 of their health (or more) suddenly vanishes.
Galio
Mana-regen problems aside, Galio is a strong Tank. His most notable skills are his Ult and his Gust -- an AoE taunt nuke is especially powerful when combined with other AoE ults, such as Annie, Amumu, Morgana, Malphite, etc. Gust is useful both as a chasing mechanic and to help allies escape. Unfortunately, opposite-but-otherwise-like Rammus, Galio's usefulness tends to diminish in matches with primarily physical damage where he can't make full use out of his passive.
Garen
Garen is not a "typical" Tank, and works best as an Offtank due to not having a strong initiator or CC other than his silence (which is pretty useful in and of itself). Garen's a tough character who can bring a lot of damage to a group of enemies between judgement and his iconic sunfire cape. His ult is interesting, as it can be difficult to judge when to best use it, though some players dislike it because of its tendency to "steal" kills.
Gragas
Gragas gets a damage reduction, an AoE Slow, Area Denial, and a massive radial knockback on a relatively short cooldown. He works well as a tank (consider starting with a rod of ages) because of fight disruption alone, and drunken rage and happy-time give him quite a long longevity as well, making him a passable pusher.
Malphite
Malphite joins Cho in having been hit with the nerf-bat in the last patch, but is still a strong tank. Like Amumu, Rammus, and Shen, Malphite can jungle effectively, and has a combination of an impressive initiator, and a damage-dealing skill that scales off of armor (so even building straight tank items, you can deal a hefty sum of damage). With his Q, he can be especially effective by tunnel-visioning enemy carries.
Maokai
Maokai is a great tank based around team fights. His ultimate is an AoE damage reduction, which when strategically placed, can turn the tide of fights. He has a snare, which is great to gank with, or focus down the enemy carry, a small AoE slow, and a really strong harass with his ranged sapling. His passive makes him have a very strong laning presence. After every 5 abilities, his next auto attack heals him 7% hp. This is also a incredibly useful skill in team fights, giving him huge survivability potential.
Mordekaiser
Mordekaiser can be an incredible tank, especially if he gets a few early kills and gets an item and level advantage over the other players. Though I've heard of folks jungling as Mordekaiser, I prefer to see him in a solo lane, where a good player can easily 2 v 1 by abusing Iron Man (well, perhaps less easily, depending on the opponents, but I've seen it done across a number of games). Make sure to get some AP to go with the Kaiser - Rylai's or an Abyssal Scepter and Sorc Boots make for a very offensive tank, especially in the laning phase where his short cooldowns make for an easy harass.
Nunu
When built properly, Nunu makes a great off-tank. The thing you have to realize about Nunu is that the slow on his E is insane. It's 70% I think at rank 5? If Nunu catches up to you alone, you are not getting away from him if he has the mana to continually iceball you. Also important to note is Consume - the health it restores is not insubstantial, and with a Will of the Ancients, you can easily recover half your health every 10 seconds or so.
Rammus
Rammus has everything a tank needs - a damage reduction ability (defense curl), an intiator paired with a slow (powerball), hard CC and target peeling (taunt), and team fight presence (tremors adds up fast!). Did we mention that he could jungle and his taunt has a relatively quick cooldown? A top pick for sure.
Shen
Shen is probably up in the top 3 with Amumu and Rammus when it comes to tanks (in my mind at least). If you can learn to correctly handle his Energy (which can be a little tricky to get the hang of at first), a good Shen player can be phenomenal. He's got a damage shield, an AoE Taunt, and a global ult / ally savior mechanic. One thing to note is that you don't want to build dodge on Shen - build armor/MR and health instead: it makes his passive come up more often and for more damage.
Singed
Finally, Singed. Singed is known for running really fast - make sure to take ghost, as it lets you spread your poison much more handily without having to path around enemies. As singed, you want your opponents to chase you; with your ult you're more than likely faster than them (and insanity potion lasts for an unmatched THIRTY SECONDS) and they'll kill themselves on your poison trail if they chase you for too long. Of course, when they finally break off, that's when you turn around, stick 'm with your adhesive, and then fling 'm behind you into more poison. Fling is an okay initiator, though it takes practice to make sure you're not accidentally throwing the enemies away from your team.
Best Answer
Depending on what level you are, you might want to hold off on buying some for a while. Here's some good guidelines for where to start, though:
Marks - Desolation and Insight are great places to start building your mark collection. Desolation gives you armor penetration and Insight gives you magic penetration. Armor penetration is good on anyone who uses their auto-attack, as it's useful for both slower, big hitters and those who want many to do apply-on-hit-effect hits. Magic penetration is useful to help your abilities pierce through any magic resist. There's not many characters that wouldn't benefit from at least one of these and many cast benefit from either.
Seals - Since the removal of dodge runes Flat armour runes are preferable for among other things jungling and supporting. Mana regeneration per level runes are a must have choice for most casters who will be spamming spells often.
Glyphs - Glyphs are tougher to define a good base set for. Celerity is probably the most all-around useful, as it provides cooldown reduction. I don't know of any character who doesn't benefit from that! Shield is an okay choice, as magic resist is useful against most characters, but I prefer the cooldown reduction.
Quintessences - Fortitude is the best all around quintessence, as it makes the early game much easier. When you have 3 Greater Quintessences of Fortitude, you start with +78 health -- that's quite the cushion in the first few levels! Once you have access to the Greater runes (level 30), Wisdom and Evasion become viable choices. 3 Greater Quintessences of Wisdom, along with the Awareness mastery net you 11% more experience and if you're Zilean, you have an extra 8% on top of that. This will give you a noticeable advantage at the end at the sacrifice of the end-game -- unless you can get yourself fed with your higher levels. Swiftness is also a good all-around, as movement speed is always part of the game.
I recommend getting a full book for one character while you're still gaining your summoner levels. This will allow you to rock out at your main character and get used to the game. Once you have a full book, you can branch out into other characters and rune setups. Read up on runes on sites like Solomid and the League of Legends Wikia Wiki.
One last tip: always buy primary runes. When you're buying your first few tier 1 runes, it's okay to go secondary on a few, but don't waste your IP on second-rate runes in tier 2 and tier 3.
If you decide you don't want a certain rune, you can combine several runes to get a new rune. Combining three will net a rune of equal tier and combining five will give you a rune from the next tier up. Other than that, they just kind of hang out in your rune book staging area