I've played UO from day 1 so 2D is what I started with, and 2D is what I still use. I like the simplicity of it. You can still maintain uniqueness, but you don't have to deal with being too flashy either.

I've tried the 3D engine, and well, it kinda sucks. It really looks like they just threw something together and pushed it out as 3rd Dawn to help sales. I don't notice a difference in lag or performance, but I can't get over the the fact that I feel like I'm playing a game that was quickly put together by a bunch of college kids as a computer science project. It looks like they tried to make it all flashy looking, but failed in the process and rather than go back and fix it they said, screw it, and turned it in, haha.