Completing Act Two (around level 40) on any class will unlock a class-specific emote that corresponds with your root class.
Completing Act Three (at max-level) as an Empire class unlocks the following abilities for everyone in your legacy: Flamethrower (Bounty Hunter), Lightning Storm (Sith Inquisitor), Force Choke (Sith Warrior), Orbital Strike (Imperial Agent). For Republic classes: Project (Jedi Consular), Force Sweep (Jedi Knight), Sticky Grenade (Republic Trooper), Dirty Kick (Smuggler). I
Maxing out affection with all five companion types will net each of your characters a total of +50 boost to their Presence stat, a 5-minute reduction on the cooldown for the super-powerful Heroic Moment ability, and a 1-minute addition to the duration of the buff that Heroic Moment gives you.
To purchase a race, which unlocks it for all class types without having to level a character of that race to 50 first, will cost you 1.5 million credits for each race. In addition, you must be legacy level 8 to buy it.
6. All the unlocks for increasing your social rank are emotes. The best emote is /doublelasers (costs 150k and requires level 15 Legacy). The weirdest is /weld (costs 50k and requires level 12 Legacy).