I recommend oRA3 or CTraid assist or similar (should be compatible) they allow the sharing of certain info between raid members (like main tanks etc) which can be very helpful. I liked it as a hunter becuase it meant I could change MD target to which ever tank required it easily without having to look through a mess of raid frames. May be more of a 25/40 man raid thing though. Not such an issue with 10 man.
f you can remember to change the settings to 'annouce to self only' Ensidia fails is a great addon. Not only does it let you have a good inication of your own performance re: standing in fire, getting tail swiped, not spreading out, not grouping up etc etc it also lets you see who else in the raid is failing and how often. If you dont have it, believe me a raid leader will and will be using it as one way to judge performance.
As has been mentioned by Unholytruth there are also addons that are nigh on compulsory for certain classes. Decursive for toons that can dispell any type of mechanic (magic, curses, poison etc) This includes MAGES!!!!!!!!. Pallypower for pallys.
Not too sure what I think about making healbot required for healers, I was in a progression guild that had serious discussions about it. The proponents saying it wass needed to improve (or decrease  ) overhealing as it showed incoming heals from other users. Detractors saying they dint like playing that way and that it wasnt responsive enough. The 1st point is really no longer valid as loads of mods now show incoming heals (grid, think you can set up xperl to as well). I think I would need to be convinced that it is needed.
That being said if you are new to raid healing, give it a go as it does make things very easy (click to heal basically).
Blizz are going to break the raid addon AVR, which (although I personally don't like it) may have been very handy to help people learn fights.
Oh and of course get a decent mod for looking at damage/healing done etc. I use Skada, a lot of people use recount. Just please don't spam it after every fight.
I would also suggest that someone DL and install a decent damage parse for one of the external sites (like http://www.worldoflogs.com/reports/9hnvo80tlo8cn25h/ - this is the guild where Repooc is btw) Very useful tool for analysing performance.
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