Healers - are you ready for raiding?
The biggest changes to raiding in Cata have landed on the shoulders of healers. Healing a raid is now a rather different beast to WotLK. It is arguably more difficult, but also more enjoyable imho.
This post is based on reading around Elitist Jerks, plus my own experience from 3 weeks of raid healing. It is not definitive, but it is intended to be helpful.
So, are you ready for raiding?
You'll want the best gear possible before you try healing in raids. This means 346 blues in the vast majority of slots, with a couple of rep or crafted epics to boost your healing a bit. Top-level and appropriate gems in every slot, including the new int + max mana meta. Enchants on every piece should be mandatory.
Some of the raid encounters are very tight for healers; I literally finish some fights with enough mana for one final cast, having used every cooldown available. Given these circumstances, every single extra stat you can get could be raid-saving, and so there's no excuse for skimping on enchants. You may think it's not worth enchanting your blue weapon, but as a proportion of your healing potential, it's actually more important to enchant blue gear than epics. We have several max-level enchanters in the guild.
Holy Paladins and Disco Priests should be aiming for 6k spellpower, with 2500 combat regen as a rough guideline. Holy Priests and Druids should have 6k SP and 3000+ combat regen. Shamen seem less clear about their stats, but I'd say 6k SP is a good guideline.
All healers currently favour spirit over most other stats. Mastery varies in value; for Priests it's pretty good, less so for Paladins and Shamen. Almost all healers have been reforging crit and haste for spirit where possible. Many of the raid fights are quite long, and many of them have periods or low or no damage where you can regen quite a lot.
Flask of Flowing Water (Spirit) or Flask of the Draconic Mind (int) are obviously mandatory. Guild alchemists can make them pretty cheaply. The new Mana potions provide 10k mana, which can easily save a raid, and so you should stock up on these, too. The best food gives 90 stam and either int or spirit; again, take several stacks.
Anyone who tells you that you can heal to your full potential without any addons is wrong. Simply put, a healing addon like Healbot or Vuhdo (with mouseover macros) means that you can select your target, select your spell and cast it faster than would otherwise be possible. Reducing the time between deciding to heal and the heal landing is self-evidently a good thing. Get an addon, configure it to your liking and get used to using it. If you have to target someone to heal them, you're not healing as well as you could, or should, be.
A boss mod is also vital. Good healers can react quickly to damage. Great healers anticipate the damage before it lands. The combination of a healing addon and a boss mod, as well as good knowledge of each encounter, can help you move from a decent healer to a great one.
It's hard for me to tell everyone else how to heal. The basic change in Cata is that your spells cost more and heal for less. Therefore, every cast has to matter. Learning the encounters will mean you know when the damage spikes come, and when there are regen opportunities. Try to cast your most efficient spells as standard, but remember you have your quick ones if people are about to die. Foremost, remember that (unlike a 5-man heroic) you're working as a team, so make sure you know who you're healing with.