1. The Feel
I don't know if it was my hands are something but for some reason the Vita didn't feel right in my hands. To me it was difficult to get them around it. Unlike the 3DS, which fits well in my hands because it's,well, a square.
2. The Back Touch Screen
I thought this was the stupidest thing to put on a portable gaming system. I already told you my hands couldn't fit around the system so it's a pain in the butt trying to touch the back screen while trying to control a game.*
*Through I never actually played a game then requires it, I assume it would suck
3. The lack of the L2 & R2 Buttons
I used to having 4 back buttons to press when playing a PS system. So when I saw the Vita don't have the extra 2, It just felt.....weird.....