Hadnt really sat down and analyzed it until now, but I actually PREFER it when someone replies with "no problem." The informality further reinforces the camaraderie or rapport established by the action that prompted the "thank you" to begin with. If I or someone else were to respond with plain old "you're welcome", to me that would instigate a certain formality that diminishes that sense of rapport. Then again, if it's a formal environment then of course "you're welcome" is the more appropriate response.