There are many nice things to day about XenForo, especially given that it's only an Alpha (since loads of features are missing).
Should I buy XenForo, the first things I'd like to change is the light yellow/light orange bars, make lots of the very light blue stuf a new color (eg. Thread Display Options), and do something with the fact that Personal Conversations/Your News Feed doesn't have the same location as What's New.
Also, Watched Threads appear two different places. IMO everything related to NEW should appear in the same, single area. Btw, the NEW icon in each post is more or less invisible. And I'd prefer to have the post date in the upper right corner instead of having it between the posts.
I'd also change how the popup text behaves (the one that describes each forum): better to make it pop up only if the user positions the mouse over a specific place, eg. the post icon.
And while I personally like the Georgia font, maybe a simple user setting, allowing the users to choose a 'master font' for the whole forum would be good?