I was just trying out Office 2013, been playing with it for a while. Aside the fact that it seems to be (much) slower than Office 2010 everything seems about okay. One thing that annoys me slightly is the cursor animation, it makes me feel like I'm a slow typist even though I'm absolutely not.
One huge thing I absolutely hate right now is the inability to choose another color for the user interface. The white is WAY too bright, especially when it fills your entire display. It's extremely uncomfortable to work with and it makes your page drown in the user interface - especially on bigger displays. I don't have it installed anymore, but the black theme in 2010 was really comfortable to work in, putting the focus completely on your content. But now everything is white, and there is no way to change it.