"If you could ask your players to change something (characters, playstyle, etc.) what would you ask them? Can that advice be made generic for all players and GM's?"
Hmm... I'd reinforce that for whatever reason, the CHARACTERS should have strong trusting relationships, and I'd ask for more work on the background/aspects to support this and more roleplaying of it. The truth is though, mostly that doesn't come up. I'm blessed with some really good role players as players, and the less experienced folks are willing to learn/explore, which is nice.
Dian