When I first head about the "encounter where we have to heal the boss" I was excited, like most healers. I read about the fight, but I was disappointed when I actually got into the encounter. I feel too far removed from what's actually happening to the rest of the raid during the encounter. It's no more gimmicky than most gimmick fights, but Holy Paladins are the most logical choice and Holy Light is ridiculously good for this fight.
Here are some do's and don't's. For more general "how-to", check out the videos over at tankspot.com and learn2playwow.com. If you can handle the slow load times of learn2playwow.com, I like that their videos show the encounter through several different UI's and have really nice explanations.
Use Seal of Light and Glyph of Seal of Light. Mana regen will not be an issue.DON'T...
Use Libram of Veracity if you have it. Again, mana is moot.
Focus of maintaining your stacks of the buff throughout the fight.
Use Holy Light.
Use Divine Illumination at least twice in this fight if you have 2t10. If you don't, use it at least once anyways before the first Dream to save on mana before you get stacks.
Use Divine Favor.
Use Avenging Wrath.
Beacon Valithria and spam your raid. Re-Beacon her every time you have to move, since moving is non-healing ("wasted") time.
Maximise Holy Lights while not inside the Dream.
Make sure your boss mod is visible and shows you the time left inside the dream, and grab an orb in the last five seconds of Dream time.
Get to your portal before it's open, and be ready to jump in.
Share orbs instead of stealing them. The range is large enough to cooperate.
Use your spellpower gear if you have it, but remember that haste is important. My FoL set is balanced at only having "haste cap" for Flash of Light, while my main spec gear totes almost 300 more haste. I chose my main spec gear for noticeably faster HLs for the most possible casts.
Remember to keep the haste buff up from Judging at least once a minute. Like Beacon, re-Judge any time you have to move and make use of "wasted" time.
Keep Hand of Freedom handy (har har) for yourself if you are immobilized by the frost debuff and have to move.
Have a macro to focus Valithria. If you /focus her before the fight begins, she'll be added to Healbot (if you use it). You will also have to refocus her after each Dream phase, so be prepared and keep the macro handy.
/target Valithria Dreamwalker
Be zoomed too far out. It can hurt depth perception inside the dream and have you fly right "through" (past) an orb.Good luck!
Drop your stacks. Three or four well-timed orbs inside the dream is better than five or six too soon and having to start over.
Run around healing people out of your range. Focus on as many holy lights in your raid as possible in your time outside of the dream. If you can, collect orbs in the front while in the Dream so you can be in front when you are out, helping heal the raid.
Take excessive falling damage from being high up in the Dream when you get kicked out. Use your mod timer and start your descent in the last second or so.
Portal jack from your other healers. Mark everyone up and be aware of where they are. Take the high road.
Cop an attitude "because Holy Paladins are gods in this fight I'm teh awezomez". It takes 10 (or 25) people to complete this encounter, and those 9 (or 24) people will remember that you're a jerk long after this fight's on farm. In the same vein, no one needs to see your meters.