For a while I played on a computer that doesn't meet the minimum system requirements for WotLK. In an effort to save cpu, I trimmed as many addons as I could. Playing on beta helped wean me off, too. Action bar addons were the first to go. They made my screen simpler, but as a keybinder, I didn't really need them. I found that it wasn't too much trouble to apply rogue poisons directly from my bags, or do crafting stuff directly from my spellbook. I didn't need an extra actionbar for those things. I found I could get away with only having addons for four things:
Unit frames. Something to move them out of the top left corner to someplace I can see them. Ideally make them visually simpler, as well. I used ag_unitframes for a long time and now use Pitbull.
Threat meter. Actually only necessary in raids or with bad PUG tanks. I use Omen.
Timer for short-term buffs & debuffs. There are lots of options, but I wrote my own addon (
NeedToKnow) so that it'd do what I want.
Cooldown notifier. Something to remind me to use my short-duration cooldowns. Eventually I wrote my own (
TellMeWhen).
Other than that, everything else is just convenient.