I hadn't heard of the Swap Mirror Class abilities idea, but I can tell you that every serious PVPer will make their alignment so that they can use the better and/or less obvious animations. In other words, mostly imperial animations, except for sorcerers who will go light side to replace Lightning with TK Throw. Commando Medics will definitely want to replace that Here I Am green beam.
My list (somtimes just agreeing with others):
Legacy Codex (should be in addition to character codex, not replace it)
True dual spec with wardrobe manager
Legacy bank tab
Legacy friends/ignore lists
Server Transfers (for the love of God, Montresor!)
Faction switch (here is the thing: if my guild and toons are republic, and lets say I have a Sentinel, I should be able to enjoy the Warrior story line and get a Guardian for my troubles).
Number one with a bullet: health text on focus target.
And number 2: target of focus target frame. Tired of not knowing who the boss is targeting as a healer.
While we're at it, mouse over healing, and smart target healing (targeting an enemy heals its target).