Ok, I'll be the first to admit I'm not terribly open to suggestions, at least that's not my first knee-jerk reaction. I'm very much of the school that says it's much easier to say "no" initially than it is to take something away later when it turns out to be unbalanced. Maybe the best way to say it is: once burned, twice shy.
That said, if a player wants something for the flavor (as opposed to min/maxing), I'm more open. If the idea is clearly tied to my campaign, versus something the player read in the latest Dragon, I'm more open. Even in those cases though, I still view the suggestions through the twin lenses of "game balance" and "fun".
I've seen too many players (myself included) push for an idea because it would give great new abilities and such, or just sounds so cool. Those things can easily create too much imbalance, which can ruin the fun, which can kill a campaign.
A similar problem comes up if a player wants to create a character with a certain flavor which doesn't fit the campaign or the group ("My character is a greedy loner who never trusts anyone. He's out for himself and couldn't care less what happens to anyone else.") I've actually gamed with players who wanted to run such characters, and it never helped the group have fun.
Like any GM, I'm always pleased when players want to take the campaign in new directions, because it means my world has begun to take on a life of its own. Such moments are a real validation for a GM. The GM's job, however, is to make sure all the players have fun, which means sometimes saying no. Hopefully, not often, but still sometimes.