I love middle school. It is my preferred age group as well. As others mentioned, each grade level comes with it's own unique challenges, but overall, the biggest challenge I see is dealing the behavior and attitude.
Middle schoolers are going through a number of changes; physically, mentally, spiritually, psychologically, etc. They are just starting to establish their own identity and most of them aren't sure what that identity is going to be, so they ARE "all over the place" at different times. Invariably, they are going to make some poor choices as they test the boundaries and limits established by the teacher, the school and their peers. Just remember that it is rarely personal. They are trying to "find themselves" and this has to include a certain amount of trial and error on their part.
I think of it like running a giant clothing store and they are choosing their own clothes for the first time. They are going to try everything from the shirt/tie combination to the dragging pants and goth outfits. I may really dislike some of the outfits they try on, but I have to remember they aren't putting those clothes on JUST to upset me. They are looking to see what kind of reaction they get from me AND from the other "customers" (peers).
It's also true they are making some life choices that may not be the best, but at this age they are still open to influence from you and may let you help them choose a better path. Once they get in high school, chances are they have already made those choices and are determined to continue following them - at least for awhile.