Personally, I think 3.x looks
very dated. It only fills half of my screen, for example. It's back from the 1024x768 era, and looks like some other (free) products.
Sites probably haven't updated because "if its not broke, don't fix it" 3.x was a stable product, back in its peak time, several years ago. They may also have a lot of customizations, don't want to pay for the current version, or perhaps the staff that installed it for them is no longer part of the picture (they may lack the technical resources or courage to do it).
I hope 3.x is sent off into the sunset soon, so they can focus more on future products 4.2 and 5.0. It seems like many developers have already done so.
I agree with some other posts that some new and innovative ideas/leadership are needed, to make the product stay at the cutting edge. They seem to be treading water at the moment, but the 4.x product is rock solid reliable and does the job well.
Keep up the great work!
Rhody
PS: vbulletin.org should probably be updated, to set the example