If you were to look at all the add-ons I have installed, you might dispute the next sentence. I don’t work with very many add-ons though. I installed an add-on pack at one time and then decided I didn’t like most of it. So here’s what I use, as a healer:
CT Raid Assist – This is for the raid frames, which I like way better than Blizzard’s default ones. I set them to show health deficits for easier healing and to prevent over-/underhealing.
Deadly Boss Mods – This is a very useful add-on for seeing cooldowns on boss abilities and the like. It displays bars for indication of timers. It also works in battlegrounds, and is very handy in AB and AV for seeing how long you have before certain points are taken.
KLH Threat Meter – I don’t really use this as I’m typically healing, but I have it anyway to see what’s happening. I will probably switch this to the newer (and supposedly better) Omen eventually, since most of the guild is.
Natur Enemy Cast Bars – I actually don’t use this for enemy cast bars. It has a built-in respawn timer for trash in Kara, Gruul’s, etc. which I find very handy. Nothing like not knowing when those damn ghosts before Chess in Kara are coming back.
Cartographer – Use this for knowing where all the gathering profession nodes are. It also apparently will link up with other guildies who have the mod and always display where they are on the map. Neat, but by no means necessary :)
SWStats – A damage/healing meter add-on. This is for analysis only of course. Though I don’t need it so much anymore with the advent of WWS…
Fontabulous – Changes the in-game font, because I kind of dislike the original. Especially the font for damage dealt. Those thin, pointy numbers are definitely lacking. However, I think this addon is no longer being developed so I’m working with a version of it from quite a while ago.
And that’s pretty much it. Like I said, I had an add-on pack at one time, so I actually have about 30 random other add-ons, but they’re all disabled. So other than those mentioned above, I’m something of a purist…I was into using a lot of add-ons at one time, but the need to constantly update them with every patch made it more work than I felt like doing.
I don’t like modifying the action bars anymore, nor the standard unit frames. (I used to use Bongos and X-Perl). I prefer a more basic raid interface that keeps track of health as well as buffs which is why I stick with CT (I had tried that Grid mod, but I didn’t feel like going through all the setup it would require to get it to look the way I wanted it to). Also, buff mods (like one that comes with CT that uses bars to show the timers) take up too much screen real estate for me to want to use.
Basically, as much as I’m a healer and am generally stuck watching health bars rather than my field of vision, I like to have the UI clear and open. I’ll post a pic later (though it’s really nothing special).
/edit: I almost forgot! Outfitter! Having a paladin, warrior, and priest, I’m running a lot of characters around with multiple sets of gear. Back when I was first starting raiding with my warrior in AQ20, I had to switch between dps and tank gear a lot. This required a lot of clicking, and I would often miss pieces. Having an add-on handle that so I can switch out an entire set of gear with only a couple mouse clicks (or even a single bound keystroke) is such a relief after that.