I hate my tendency to get svcked into unproductive arguments and such: most other people seem to be much better at walking away from such situations but I find it takes a conscious effort for me to do so, and even then I often dwell on things instead of letting my mind occupy itself with more pleasant and productive thoughts.
My body's tendency to not want to lose weight is also annoying: dieting usually just makes it want to shut down, which is frustrating. I probably need to feed it (much) more caffeine, that'll learn it.

Edit: I also wish I was a better writer, especially from the perspective of not repeating the same words often: did this short post really need four repetitions of "tendency"/"tends to", for example? Bah.