Carbonite is a must for the map alone. Have a map of the whole battlefiled that shows all of your allies and who is in combat and with whom. A list of nearby enemies also pops up and you can target them by clicking on the list items, which can be helpful for targeting enemies you can't see, or who are behind a screen of defenders. Excellent for targeting the EFC.
Deadly boss mods pvp mods will put the names of the fc and the efc up on the screen too, so you can always know who has the flags.
If you are a healer, then you will prolly want Grid, Grid2, or Xperl Unit Frames sp you can see and select members of your raid, and not just your 5 man group. Xperl sure is purty, but I found xperl to be buggy, so I use a Grid2 on my priest. The raid frames show health, curses, diseases, CCs, etc., and spels you have on them like renew and shield, with a really intuitive color change to indicate a cooldown until you can shiled them again. You click on the raid frames to select your allies and cast heals and dispels on them, etc.
Bartender let's you customize youraction bars, resize them, put them anywhere on the screen. I have a row of clickables next to the Grid2 raid frames for things like bombs that just have to be clicked.
If you are a melee class, then you will need a swing timer for jousting. I forget the names of the swing timers.
That's all the addons I can think of right now that are important for pvp at 19.