I saw this thread and needed more than a few seconds to go to it.
The most offputting trait in a boss is arrogance. There is a difference between arrogance and confidence. Arrogance means you think the people that work for you are the "plebes." They forget that, without these people, things wouldn't get done and they couldn't sit on their lofty perch. Yes, I know you're not supposed to be buddy-buddy with your subordinates, but sending out signals that they are a notch below is toxic.
Even though I took more of the number-crunching courses when I studied business, the "soft" courses about organizational behavior and human resources are eye-opening. At opposite ends of the spectrum lie the hierarchical organization and the matrix organization. I found that I work much better in a matrix organization, which is a community of peers and you are there to get the job or project done without a lot of formalities/titles/red tape. BellSouth (now AT&T Southeast) was hierarchical...people were too busy worrying about titles, kissing ass, and posturing to move up...and some of them had neither the know-how (neither practical nor academic) to be where they were.
A second trait would be incompetence. Let's face it, there are people in high(er) positions that have their jobs because of numerous inadequate reasons. Eventually, most of them are "smoked out," but many people are inconvenienced until this occurs.
In my specific case, I would say that I've run into some scrapes with bosses when there was not much of an age spread and I had more credentials and/or had gotten my license prior to them. They appeared to be threatened and were way too defensive. I would say that I've liked it better when I've been sort of a "mini-firm" within a firm, where I've had projects assigned to me, either solo or along with a few people, and we just get down to work.
I think I've been fairly well-liked as a supervisor. At least, that's what I've been told (and one of the people working for me e-mailed me the link to "Canadian interview gone disgustingly wrong.") Most of the time, I was at the same level as my co-workers and got promoted because I was fairly organized...and they didn't want that level of accountability, so they didn't resent it. But it's weird going from the lunch time pizza/once-a-month-Greek restaurant or happy hour thing to being someone's supervisor.