The quick answer is, I like it. I like it a lot, the interface is very clean and is a very fast application, it seems that it is going on the right direction and I feel very comfortable with it.
But there is one thing that really makes me think what were you guys thinking?. I have been a developer for 8+ Years and I am very proficient in front-end applications and I can tell you right away when something is done right or not done right, and I think in the over all , you have done a wonderful job on the first beta release, and I am sure it will only get better.
Ok so going back to the topic, what I really dislike about Xenforo is the the addon development framework. It is honestly not really coder friendly, and it requires a lot of front end interaction, and user interface interaction. When I know very well everything your addons do can be done from the back-end solely. And the main reasons I think that, is because it asks for references to classes in the admin front-end, so the motive I am sure is not to make it more "user friendly" and at the same time coding; if that is what you thought, it is obviously not right. When if I user doesn't have any idea how a class works, they would probably be lost on the spot.
So, as suggestions I would say, how about make the front-end switches and inputs that addon usually require, into a HTML renderer class, that can produce elements such as those; spin-boxes, inputs, check boxes, etc. I think this would make it a breeze, and give developers a brake also. I mean, I am not saying is too hard, but I am giving a general prospective of what I think, and giving you feedback for it.
All these, references, callbacks, function/methods overwrite, filters , all that stuff could be done from the back end,without having to go into the administration. All these elements on the admin should be able to be produced/renderer also from the back-end, just about everything if not everything.
Or at-least not replace the current addon system but, should be able to do it, right up from the back end as well. It slows down core coders as how it is now, and these that like to go straight to the point.
I hope my feedback matters and that you don't think I am crazy