Creative control.

Welcome to the Forge, Duke.

First, this is a topic for the publishing forum, most properly, not game design. A moderator will surely move us out there at some point.

Second, perhaps you should sharpen that question somewhat; opinion polls are frowned upon for a reason, and that reason is that asking my opinion about creative control is much too vague to garner any kind of response. Do you need the legal angle? Or advice in a concrete situation you're undergoing? Or abstract moral principles for future use? How are opinions going to help in that? Some of those possibilities go far out to the realms of ethics instead of rpgs. Shortly put, my opinion about creative control is "Cool", and nobody is interested in that.

Anyway, assuming that you're interested in the legal angle: rpg material created in a session is assumed (by me, that is; no judge I've heard of has published an opinion) to be a shared creation of each participant, unless there's some other agreement in force. The very least other participants can claim is that the character etc. would have to be stripped of any of their contributions (if that is even possible), which for a healthy game doesn't leave much.

However, the situation is further complicated when you consider the rights of the original creator of the game; if the character, setting or whatever is a derivative work based on a rpg by some other guy, then that other guy can also limit publication of the character etc. Prime candidates for this situation are various D&D homebrews, which usually are chock full of exclusive Wizard's IP, from recognizable artifacts to original D&D monsters and gods. And if there's statistics included with the character, then you might break fair use (which usually protects statting stuff) if the system in question requires you to include proprietary material in the stats (Warhammer Fantasy Battle might be like this, not sure).

The simplest approach to the matter is to simply take the focal idea of the character or whatever and strip away anything somebody else might have created. Usually it's sufficient to remove names, actually, considering the level of originality you typically see in rpgs. Or get a permission from the other folks, that's not usually a problem. Also, remember that you don't need a permission if you're not going to publish the stuff: you can use anything at all for private purposes without a licence, which is why it's legal to play a game set in the Matrix universe even when there's no officially licenced rpg for it.

Also, a special case is gaming material that's created between sessions. For example, if a GM creates a setting before starting the campaign, then he still owns the setting after the campaign. What has happened is that the campaign in question is a derivative of the setting. In this case the GM is in position to stop other players from publishing any material from those sessions that uses his setting, unless the setting was changed in the creation process enough to stop being a derivative of the original setting (what's "enough" is totally up to the court). In this way different players might own characters, setting or other ideas that go into the pot at the start of the session, and their rights are not ignored just because their creations are used to create something new.

For a closure, I'd like to note that the GM has no special legal position whatsoever as concerns the group's creation. Assuming that the whole group participates in the game, then the whole group is the "creator" of the game session. I don't know where that idea would come from, unless a particular GM runs a very domineering game. It would be very difficult to verify after the fact who actually creates anything in a healthy rpg group, with everybody throwing out ideas and riffing on stuff others bring up.