I don't have much IPB experience but I've been impressed with the search and the tags so far. The similar threads feature seemed to work well too.
A design element which is simple and effective was the Ask! Tab. [Demo: GBAtemp ]
I think this could be replicated with xenforo easily.
Use Jake's Nodes as Tabs to make the Tab, and then some forums for the question categories, and then a template change for changing [New Thread] to [Ask a Question]
They also have some version of "Alerts" which I thought was pretty good, not as good as Xenforo mind you.
Their arcade has Angry Birds !