I mainly play on the Empire side for a few reasons, the majority of them are vanity reasons.

The first and biggest would be that my Sorcerer was my first 50 toon I had achieved and because of that I know more people on the Empire. The second is the animations, the animations for the Republic characters feel dry and empty to me, they lack the visual flare (I especially love lightning from sorcs ). The third reason is playing through the Empire story multiple times and only playing the Republic toon once, I noticed there is a lot less corruption in politicians on the Empire side. I don't necessarily agree with the whole genocide hysteria the Imps have, but I appreciate the lack of corruption. I am also beginning to think that the people of the Empire themselves and not the Sith are not all that bad. Throughout the agent story line its the Sith and other force-users who seem to cause a lot of the problems the galaxy faces, take away the Sith, and the Empire is actually full of good people. Finally the biggest vanity reason is the ship design of Imperial ships. I hate the Republic Hammerheads more than anything else. With the more classic Star Destroyer designs of the Empire, I like to play them more.