A smug sense of self-satisfaction.
No, you don't get anything at the moment. I think if you kill too many lowbies you get a temporary penalty but that's it. I hope that doesn't change too soon, but I fear it will. In my eyes the idea of open world PVP means you can kill anyone anytime. I think they've struck a good balance right now with the outlaw system.
So why roll PvP? Why is it fun? For me it adds an extra layer to the game. Call it player generated content. A band of high-level assholes group up and decide to terrorize Noobytown, and a guild of white knight types (Like BELIEVE) arrive to pvpwn them. Two players are both farming the same area, and one decides to eliminate the competition. You're riding along and see a player a few levels below you with a red name. You don't know who he killed, but you know he's a killer. And you know you have the means to exact sweet vengeance for your unknown fallen brothers.
During one of the closed betas the PVP server went down, so I worked on my character from CBT1 who was on a PvE server. I immediately realized how much I missed the element of danger from the PvP side. On the PvP server I tried to stay off the beaten path to not make myself a target. When I was killing mobs near another player, I kept my eye on them. And once I leveled up a little more, I stopped running and started fighing. And winning! Meanwhile on the PvE server I just quested. Kill X whatevers, turn it in. Repeat. Repeat. Repeat.
Yes, sometimes the assholes will do some damage. I saw two players performing a long boring escort quest outside of Crescentia only to get pvpwnt right at the end of it by a pack of gankers. I was too low a level to help so I made like a tree and got out of there. Another time my mystic and a warrior were two-manning a BAM when a ganker slayed us both. Oddly enough he killed the BAM too and we still got credit for it.
Last night some elin bastard jumped my archer while I was dealing with a group of mobs. I managed to kite him and the mobs long enough to take him out. I had less than 5% HP when he finally dropped. There is nothing better than killing your attempted killer.
So there are pros and cons but for me the good far outweighs the bad. Especially in TERA, where the PVP is just so damn fun, I can't fathom rolling on a PVE server. I'm obviously in the minority, though, so take that with a grain of salt.
Edit: And I should mention you can easily mitigate the damage if you're being terrorized on a PVP server by flipping to another channel. Unless you have encountered a really persistent ganker, it's unlikely that he'll jump around all the channels trying to find you again if you switch.
