Note that this has been edited to reflect the changes in Cataclysm / The Shattering, so the stat weighing is not final and a proper raid-balance of stats is not concrete.
Int provides you with spellpower and a larger mana pool, so it is naturally a strong stat.
Haste is also important, as it allows you to cast your spells faster, and to an extent, reduces the GCD (which makes a difference for instant cast spells). You get a good amount of haste passively through talents though (9% through Judgements of the Pure, 3% through Blazing Light, plus flat cast reductions on certain spells).
Crit is always nice to have, and it synnergizes nicely with some talents (Crit Holy Shocks reduce cast time on HL / DL, for example).
Mastery (at 80 and beyond) gives your heals a bonus shield equal to a % of the heal amount on your target. Shield effects have always been quite effective, and this more or less gives you a permanent Valan'yr effect to work with. I can see this being a very popular mastery, however we'll have to see how well it scales further on.
Spirit is now the go-to stat for mana regen as of The Shattering, and will continue to be so throughout Cataclysm. There may be a FEW items / enchants out there that provide MP5, but for the most part you're looking for spirit for your regen needs. It still remains to be seen just how much spirit is going to be needed to sustain throughout hardmode fights (especially with the ability to Judge Seal of Insight AND Divine Plea as decent mana regen abilities), but regardless, Spirit is the new regen stat.
As for leveling, just worry about Int, Crit, and to a lesser degree, Haste. Early on and leveling, a lot of healing will be done with Holy Shock and Word of Glory, which are both instant. Haste wont play a big role until later when you need to utilize your Flash of Light/Holy Light/Divine Light spells a bit more.
OK, questions has been asked many times.
Here is what you do while leveling:
Do Battlegrounds, as A) It gets you honour but B) It now gets you experience points for completing objectives which means your "killing 2 birds with one stone" as the expression goes.
DO NOT spend your Honour Points whatever you do. Wait until you get to level 80. Even when you get to level 80, do not spend them, as you wont have nearly enough for what you need.
Once you reach level 80, there are several things you need to do. Firstly I would suggest not going to BG as you will get destroyed. Instead doing your HC's and getting PvE gear. Once you have decent PvE gear, you can start doing things Like VoA which allows you to gain your PvP gear. Also with decent PvE gear you can half decently play PvP BG, although with no resilience you are bound go get done.
Once you have accumulated the Honour Points you need, start buying your PvP gear. First of, you want to get the set bonus that gives you 100+ resilience, this is two items. If you want to do PvP you need to get this quickly, therefore you need to get the cheapest two items, (normally shoulders and hands if I remember correctly). Once this is done you are good to keep adding, once you have your 100+ resi, you'll find it much easier to do PvP but not that wrathful gear is out, our still going to struggle.
Additionally what you want to do is gain Sons of Hodir reputation to allow you to get the head enchant for when you get your head.
REMEMBER: Always gem and enchant your itmes with the best you can possibly get as soon as possible.
Best Answer
Ahn'kahet / Gundrak / Drak Tharon Keep has dropped good gear for me as for my Tank/healer specs. You can try Halls of Lightning in Storm Peaks but best go with very well geared party into Ulduar. So, Storm Peaks is perhaps the best. Nax is the entry level raid instance for new 80's. You could also try and get into a Sartharion raid which you can get to via Theramore on Kalimdor. I didn't really do Naxxramas and skipped it. Went back later for the achievements. Lots of fun in a 25 man raid though.
Basically a good raid party that is well geared can help pull you through the instances.
But looking for the rare gear, you can consider trying to get the Tier 9 gear. This guide is not bad either. A lot of good gear can be acquired via badges. Ultimately you are preparing for level 80 raiding in 10 man dungeons. This guide is a good for an idea about which instances to use for raiding, (it is geared for pallies, but it's good for any class preparing for raids.).