Before going in, I'm expecting reflections. It's always reflections. Also realistic hair.

I remember shadows being a tad annoying too when I did some 3D graphics programming, but I was probably just crap. Collision detection and the camera are always fun too.

So uh, basically everything lol. Edit: turns out that's actually the title.

So first up they say doors. Makes sense since you've got portal culling and shit like that.