The usual: left lane campers, slow drivers (who always seem to speed in parking lots and neighborhoods), people who weave through traffic. Red light runners are the worst, especially the egregious ones who still try to run 2-3 seconds after the light changes and other cars have clearly entered the intersection.
I will admit I can be an aggressive driver. On main roads I'll generally go 5-10mph above the speed limit and on the highway I like to go at least 80-85. Speeding really doesn't make much of a difference in your ETA, maybe a few minutes. But I just enjoy driving faster too.
However when there are a lot of people on the highway and just miles of cars ahead (as is often the case in the LA area), there really is no point of getting ahead of somebody in the left lane just to get stuck behind another.