While the XFL wasn't NFL quality football, which is what Americans (and whoever watches American Football) are used to, the XFL introduced things that are now used in the NFL and other sports, such as the camera that hangs over the field, as someone mentioned. Cameras in the locker rooms would be another thing that comes to mind... I don't recall ever seeing a camera in a locker room before the XFL did it.
WWECW was never the "Paul Heyman ECW," but served as a good enough "C-show" for WWE that SyFy ended up putting SmackDown on their network. I think part of the reason it has gotten such a bad rap was simply because it was called "ECW." I doubt anyone would have had an issue with the show itself, if it were named "WCW" or something else that didn't tie it to "Extreme" wrestling.
While WWE Studios hasn't put out any "Blockbuster" films, pretty much limiting their films to a handful of theatres, before going straight to DVD, it serves as a platform for them get their names out there outside of the wrestling ring. Also, without WWE Studios, I don't know how feasible a "WWE Network" would be, which is what they obviously are aiming for.
That leaves the WBF... Which I can't think of a single positive thing that happed due to it's existence. The only thing that I can even think of that happened due to the WBF was getting Lex Luger to sign with Vince. If anything, the WBF probably sped up the process of the Federal Government indicting McMahon for steroids.
So, while McMahon has made mistakes, I gotta say the WBF is the worst "creation" that he had. While the other things effected the world that we live in today in one way or another, the WBF's effects were pretty much all negative... Unless, of course, you're a big Lex Luger fan...
-Bill