I just renewed my license and I've installed it on a testdomain, I'm currently using vBulletin 3.8.7, which is the forum I will upgrade if XenForo let's me do what I want.
1. Creatinng a unique color scheme - I think it's hard to get it right, it's not clear at all which colors belong to where. Gradients are all over the place, which does't look good using a dark scheme, I can remove them or make my own, so that's not a big issue.
2. All the popus windows - At first it was cool, but after a while it get's annoying, also the blurring effect when logging it doesn't look good using a dark theme, I would prefer to disable all the popups and special effects, I don't need them and they are not necessary in my opinion.
3. Sidebar - I know it's probably there to make XenForo unique from the others, but I prefer to have that info at the bottom, It's not important to me who's online or who's the boss at the forum.
I think that's it for now, please let me know your thoughts about this, maybe I missed something