I really have to disagree with that, it was. The entire thing is practically OOP, so if you want to stick a version on it then 5.1 with sprinkles of 5.2 .
Back to the point, IPB serves very well from a user's perspective. The hardships that come from working with IPBoard is that we confine ourselves to the hooks/application system and vow not to touch the core code. If we broke free of those vows we could start modifying the forum software to how we see fit. The only problem is that we're used to some of the niceties other frameworks provide, which is why from a developers point we see them as behind the time.
As for moving forum software, I am all for waiting until IPB 4 is released, so we can then make a choice based on the direction IPB moves if we should continue using it.