If I were to sum up the major differences between vB and most other forum softwares, I would say that vBulletin is a deeper, more robust front and back end software, whereas most others take a more basic approach.
This is good or bad depending on your personal likes and dislikes. I used to kid a friend of mine who just loved Fords above and beyond everything else, by telling her the reason she loved them so much, was because she really enjoyed having something to do on the weekends (i.e. Fixing whatever was wrong with her Ford that day.)
Though I'm not saying vB is a Ford, I'm making a comparable metaphor. With vBulletin, your either going to be intimidated/confused/aggravated with the controls, or your going to love the absolute depth of them and how you never really know everything there is to know. For me, that's really why I love it. Administrating vBulletin has taught me more about coding in general then anything else would have. This is not just in due to the general maintenance required of vB, but also due to the fact (and I don't think I'm exaggerating here) that vB has far more modifications and articles devoted to expanding your board then any other software. Just learning what to do with templates alone, can teach you things about coding you can use in other web based design.
Other softwares, at least the ones I've experienced, take a more basic, bare-bones approach to administration. And some people just don't plan on delving into their software that much, or are just not that interested in dealing with a perceived higher learning curve.