Two more things, replies to people in this thread.

AltairAntares - I really do agree with you. Anticipation is the most important skill a healer can have, and preparation is the best skill a disc priest can bring to assist the raid. Shielding people before a huge blast to get a huge credit on the healing meters will limit what other players have to pick up.

Twill - I agree with you, and this is the counter argument I've turned over in my head many a times when I wondered if I should keep being a passive healer (like I describe in my above comment) or join the club. Everything I don't go after, someone else has to, and that diminishes the mana in one part of the raid faster instead of evenly across all the healers. In my personal experience, however, I found that it was still better for me to play like I described so that when the players competing for meter performance didn't have to make hard choices between juicy AOE healing on all of group 5 and saving a random rogue in group 2 who would die. By prioritizing people who need my heals, the rest of the healers inadvertently get to have a bit more fun because I'm covering their asses when they're being greedy. When I can't cover everyone, that's when the raid leader does and should yell, and you have to reel in the meter healers a little. It's probably something that works best in an environment with the same people over and over, you know if you can trust people to ignore that group 2 rogue or not, etc etc.

I guess overall the best thing to do is find a balance. Know when it's right to heal like this, and know when not to.