I guess I should ask what does "mean" mean?
Does that mean "yelling," "someone who wants to rip your head off" "someone who is always snarling"?
And what is a nice guy on the field, for that matter? Is it someone who helps you up when he tackles you? Is he someone who will go easy on your if he knows you're in a vulnerable position?
I really could careless if they're nice or nasty as long as they do their job. Some players are very nice off the field, but when they get on the field, a switch flips and they put on their game face.
I think the "mean" vs. "nice" player is overblown and, IMO, a way to demean players with nicer personalities (sort of the "nice guys finish last" kind of mentality prevalent in our society today).
Note: I am not saying that's the perception of anyone in this thread, just my perception of the general debate.
Reggie White was a nice guy off the field, but when he was on the field, he was an intimidator. So was Dick Butkus, who is really a nice guy off the field.
And do I even need to mention Walter Payton. Had the highest pitched voice and was one of the nicest men in football history. But the way he played was quite intimidating, even as a running back.
Now there's a positive side to "appearing" mean. I think you can see the results in the Jeremy Stevens-Joey Porter hype before the Super Bowl.
Joey Porter came off as "mean" and, IMO, intimidated Stevens even before the game started. But that may have been more Stevens insecurities and not being a focused player than Porter's intimidation. But it worked on Stevens.
There's also a downside to the "mean" player, if by "mean" I mean a player who wears his emotions on the field. That player can be baited into a penalty or a bonehead play.