I'm a tank & that is all i do, still learning the Cata Raids & Heroic Dungeons.
I run with about 3 add & I'd recommend them:
Taunt Master: Allows you to quick taunt off threat from aggro hors. Also provides some quick access to secondary tools like intervene, intercept, and heroic throw when taunt isn't refreshed
GTFO: Basically sound the horn when your standing in bad stuff. Nice, especially when the stuff on the ground isn't easy to see.
Recount: Most have this, but use this in tanking for "damage taken" tells you why people died so you can correct for the next round.
*Optional is a real time threat meter if you want.
I stick to the old exp to 26 and hit to about 7.2%. I don't keep it at 8%, but put it in a place where I can use the dragon flank that adds stam/hit so i'm up to 8% when time comes (not yet). The rest goes to Stam/Mast & Dodge/Parry as needed (or as the socket bonus gods demand).