I think the main thing with playing a healer that I forget (and this happens across all my characters) is to watch my OWN health. As a tank, it's not my job (it is though, I've got cooldowns, but as a non-raid tank, it rarely matters), and as dps I'm situationally aware (don't stand in fire!) but otherwise barely watch my health. (Even as warlock, I lifetap sparringly now, I rarely run out of mana anyway.)

But seriously, as a healer in dungeons, it's easy to get focused on healing the tank, and throwing heals at the other little bars, and forget that you take random damage also. Many times I've suddenly noticed being real low.

(Not like the one beetle guy in OK, where the fucking worthless tank didn't get agro on anything but the boss and all teh adds were chewing on my little priest. I went oom healing myself in that fight, the tank barely taking any damage, and I was the only one that died. Quit group after loot with the comment "sorry, I didn't bring my tank set" but the asshat DK probably didn't notice or care.)