Best - The damn dual analog sticks. Feel so natural. Also the form factor of the device is great - I'm not getting cramps like I usually do from handheld consoles.
Worst - Most games run in sub-res. I don't know if it's just the fact that the games were rushed for launch, or that the Vita's GPU just can't handle that resolution, or that it just can't handle the ridiculous shaders and effects they're trying to pull off (see Uncharted) but I didn't buy a Vita for blurry, jaggy games. I don't even care if they don't use AA (even though the Vita's GPU seems to be adept at this) as long as they hit native res.
Games don't need bloom, parallax mapping, and ridiculous poly counts to be fun. They DO however need to not offend the eyes of the player and perform at a steady framerate. When you try to run fancy shaders, but fail to up the resolution, you just make things worse. Where the jaggies on the PSP would blend in because they didn't use things like rim lighting and specularity, which make the rough pixels stick out like a sore thumb.
Sorry for the rant. And before someone calls me a graphics whore, note that I would be fine with playing a game with very low poly counts and no normal mapping as long as it ran at a constant 30 or 60 fps and didn't have terrible aliasing and blurriness from being upscaled.