You can't be the "nice guy" all the time, moderator or not. Don't even try it because it'll make you shapeless. What does nice guy mean anyways? As a moderator you'll simply have to moderate in a moderate way. Everything else makes you a looser mod.
Opinions differ, and will especially when you're going to close threads or point out "misbehaviour", the point is: are you going to make it the ******* way or will you be cool?
Oh, surely it doesn't made any sense, but hey,.. I am directly behind the last one on the list of who's next to be a mod.