I've been used to phpBB until more recently when I've been having a go with vBulletin.
It's obvious that VB has a lot more functionality and is "better" in many ways. I had a vague thought that I might get it for my place but we had a somewhat turbulent time with a revamp (still on phpBB) we did a couple of years ago and it drove our users nuts.
In many ways I think it's better just to carry on with our clunky old thing - it's what they're all used to.
As a site visitor (not just forums) I think I'm also quite resistant to change. As a member of several affiliate networks, it makes me crazy to go there and suddenly find that the user interface has completely changed. Oh, yippee, another new thing to learn.
And I bought a new camera recently but none of my existing software worked with it. I didn't mind buying the upgraded software. But that's yet another new thing to learn.
It sometimes seems as though everything I learned yesterday is obsolete today. It's very hard to keep up. I know it's all "progress" but the rate of change is getting too fast for me.
I think it's maybe better not to thrust radical change on your customers/visitors/users.
How do you find the balance between progress and people being unhappy about change.