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Why are characters banned from DotA-likes in tournament play during selection? Are there characters that are actually imbalanced; why not just balance them?
If it's just for the sake of another level of meta-game strategy, then where did it come from?
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.
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'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)
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.
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 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 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 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 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 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.
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 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 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.
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.
There are virtually no differences. It's almost an exact copy of the original Warcraft 3 map, minus the improved graphics, down to the very level of the basic controls itself (animation cancelling, etc.).
The biggest difference that you'll feel while playing it is the matchmaking system which Dota2 will take from games like Team Fortress 2 and Left 4 Dead and a ranking system about which we don't know much yet.
I'd love to give sources on this, but I'm afraid I don't have ones on hand right now. There aren't exactly a ton of interviews on the game, and most of this stuff is gathered from simply watching the game (it looks like it's an exact copy) and posts by icefrog (one of the original Dota caretakers who now works on the game) on various forums.
Best Answer
No, this isn't related to balance, and is actually strategy related.
The origins of banning heroes comes from a match type called -CM, or Captain's Mode in DoTA. In Captain's mode, teams take turns banning heroes they don't want the other team to use whilst picking their own heroes for the match. One person from each team (the person in the first slot) is the one who makes these decisions, so you usually only play CM with a group of people you're familiar with since it requires a lot of coordination.
In professional level play, this becomes extremely important as you want to prevent the other team from picking heroes that they are adept at, or countering the heroes you picked.
For example, say you wanted to go with an "all push" strategy that relies on pushing down lanes to end the game fast (with the drawback that if you don't end the game early, the other team will easily overpower you over time with their "late game" heroes). Your team will probably want to ban any heroes that can counter these pushes, or easily kill their heroes.
While balance/imbalance can certainly have a factor from patch to patch, professional level play sticks to a version of the game that is considered "stable" by the community.