"Player A is a combat monster! All he does is hack and slash; all he wants is to steal treasure and gain experience points. How can I get him to roleplay?"Sound familiar? Or perhaps:
"Player B thinks he's a tactical expert. He refuses to go into combat unless we have a detailed and exacting plan. But I've got this sweet plan to change his ways ..."
And let's not forget:
"I don't even know why Player C brings his girlfriend! She's clearly not interested in what we do. She never speaks up, volunteers, or even tries when we come to a crisis situation."
Who's at fault in the above examples? The speaker, of course - for trying to force someone into a role they don't like.
The end-result of a role-playing game is for everyone involved - players and game master alike - to have fun. Different people have fun doing different things. It's no one's place to say "your way of having fun is not allowed" - not even the GM's.
"But so-and-so is being disruptive!" That's a problem, sure, but why is that? Aside from active malice or testing the GM's cojones, players are disruptive because they want attention. They want attention because they feel their needs aren't being met. 9 times out of 10 it takes remarkably little to soothe a distracting PC.
Players aren't problems - players who aren't having fun are problems. And it's easier to solve that then you'd think.