Again, not a massive step up in difficulty for most people (ie not healers). For DPS and tanks, business as usual, it is just that everything hits harder and is harder to kill. So your priority order and ability to burn down adds as fast as possible is on the line here. Make sure you have allocated your left and right teams well enough to cover off managing adds. Also note you will be 1-2 DPS down due to increased healing requirements.
In heroic mode there are two major changes to Valithria - first, she loses health over time, second, going into the portals is damaging to your health and far fewer balls spawn in the dream. Oh yeah, and they are red! Because RED is BAD MKAY?
Anyway, you will need about 8 healers for Valithria - 3 raid healers to stay outside and keep the raid going and 5 dream healers. All healers should be putting any spare GCDs on Valithria as well as any spare AoE healing. The dream healers need to allocate themselves one eacy to a portal (when they appear) and edge their way toward them and stand next to them as soon as possible. While outside, dream healers should be helping with AoE healing on the raid, but their number one priority is healing Valithria.
Once in the dream, it is quite likely that there will be a red blob above your head. Go and grab it to refresh your stacks. Then, find a fellow healer or two and fly around together bursting red bubbles. It is really important that you move in groups of at least 2 more if possible so that the most people get the most stacks. The red blobs REALLY are thin on the *cough* air and you need to share them to get Valithria healed. As long as you are within the area of the explosion when one is popped you get the (de)buff.
Just like in normal, get your stacks up as high as possible, and heal the pants off valithria. Heroic is just plain Harder, so expect a few wipes while you find your groove. Good healers to go into the Dream are pallies and shammies, priests and druids are probably better on the outside. Save your CD heals for just before Valithria is healed and your stacks are high.