I dislike Ace2, for actions bars I use Bongos 2, the best!!!
I get all mine stuff from Curse.com...
Bartender 3 is what I use for Bar mods, I used to use Bongos but it wasn't Ace2 compatible and it has all the same features and functions of Bongos.
Heres what I use for mods that I would deem neccessity.
Omen & Threat - makes sure you don't pull aggro off the tanks.
BigWigs - If you raid it's an absolute must as it warns you about Boss events.
LittleWigs - Boss info for Outland 5man instances
Quartz - For any caster, let's you know when you can start casting the next spell based on lag. This is for the best dps & casting efficiency possible. It also makes tracking incoming casts much easier to track.
Use in conjunction with the macro command /stopcasting...
Ex macro..
/stopcasting
/cast Mind Blast
The /stopcasting command will not be necessary in 2.3 patch as it will implemented in servers when you press, click a new spell.
Class Timers - Tracks all your DoT's.
In my opinion these are absolute musts for addons, any others are your personal preference.
I use...
Bartender 3 - Bar mod
Prat - Chat mod
Freerefills - Auto buys food, drinks, reagents when I visit vendors.
Automaton - Automatically does common stuff for you.
DurabilityFu - Auto repair and % armor damage tracker
Reagantfu - small fubar addon that shows my current amount of reagents
DPSFu - tracks my dps
Mobhealth3 - Gives numerical values on mobs health
XLoot - Compact loot window
XLootGroup - Compact item roll windows
EEPanels - lets me customize my UI
XPerl - Unitframe addon modifies the standard untiframes, gives more useful info
Examiner - lets you check out ppl's gear and shows many useful information enchanced inspect basically
Auctioneer - Easy AH selling & buying.
AtlasLoot - rep items, pvp item, boss drops, etc...
Cartographer - Enchanced Map function
Simple combat log - Easy to read combat log info
Simpleminimap - modifies the minimap
QuestFu - Questlog tracker
ClosetGnome - Easy 1 click switching between armor sets
I have a few other but these are my main ones.