I'm looking at playing a Slayer in the coming Open Beta, and I was hoping for some feedback from people who had played more than I have in the Closed Beta, about how the racial abilities synergize with the Slayer class. I understand how all the racials work, I'm more interested in the intersection between said abilities and the game play style of the Slayer class. Specifically I care about PvP (though, other domains are important to me).
In general, the racial skills in TERA aren't meant to be super-critical in your choice of race.
That being said...
None of the other races synergize very well.
In the end, I'd choose an aman or a castanic. If you're going to focus exclusively on PvP or PvP is all you will do end-game, then Human is not a bad choice either.
Adding onto @Shaun's excellent answer, there's one more thing to consider: hitbox.
Playing as an Aman, you're one of the largest classes in the game. This is excellent for tanking damadge, as the larger your hitbox, the more that you have the chance to block. It also makes it easy for your priest to heal you, as you're easier to click. But for PVP, you stand out like a sort thumb.
And vice-versa for Elin.
Aside from hitboxes and racial abilities, the only other deciding factor of class is aesthetics.