I am starting to develop a home-brew system along some members of our group, and one of the things we have to decide is the advancement system. We are divided between point-based and class/level, having experienced and enjoyed both of them. After reading this question about the differences between those two, I stumbled over this other one, where the most common flags of the point-based system were discussed, and now I want to make the same question about our other candidate.
The first question linked contain some interesting answers, but those are more centered on players and DM interaction with the system, and I would like answers more focused on the issues on a game design level, like those in the second question.
Concretely, I would like to know the most common pitfalls in those systems that would encourage players playing the system instead of the game, aka powergaming.