Felix said:
Who then determines what the specific result of a successful check is?
If it's the DM, it's still DM fiat, and you've changed nothing.
If it's the player, and the DM has veto over the result, it's still DM fiat, and you've changed nothing.
If it's the player, and the DM has no veto power over the result, then you've taken control of an NPCs actions in much the same way that a DM might take control of a PC who failed a save vs Charm Person; in the same way that a PC should not be taken control of by the DM, neither should the NPC be taken control of by the player.
So which is it?
Well, if I roll my to-hit and damage and score a hit and enough damage to kill the target, I've taken control of that character away from the DM, haven't I? This is not an all-or-nothing issue. It's a question of degree. The social skills
assume that the DM will need to reliquish some control over the characters on which those skills are used. That's their
raison d'etre. If you couldn't exert some measure of control over an NPC using Intimidate, Gather Information, Diplomacy, or Bluff, you'd have no reason to put ranks into them.
These skills were designed to allow a PC to influence an NPC to a certain degree, and the degree to which that control can be exerted is more or less spelled out. You can almost, but not quite, duplicate Charm Person using Diplomacy by changing an NPC's attitude to friendly. Charm Person changes the target's attitude to friendly, despite circumstances, and leaves it there. Diplomacy takes more time, more effort, and can be ruined by circumstances: "yes, I killed your wife, but please don't hold that against me" will probably not work with a Diplomacy check, but a Charmed character will have to forgive you for the duration of the spell, because he can't stop being friendly.
Another important point is that Diplomacy only works on NPCs, presumably because, as you say, the DM shouldn't be taking control of player characters. It clearly says in the description that it affects NPCs. The "demoralize opponent" version of Intimidate works on PCs, although one might argue that the more general use does not because it duplicates the effects of Diplomacy.
It's no fun when your character gets puppeted by the DM, but under certain circumstances it's valid under the rules. However, these circumstances have been reduced compared to what the PCs can bring to bear. There is an imbalance built into the skill system to help protect player autonomy: PCs are better at social skills than NPCs, by virtue of being PCs. If the DM chooses to ignore the skills' description of what it says a character can do, and he hasn't discussed this with the players ahead of time, that's unfair.
I'm always arguing this same point: if the DM is going to overrule the action of social skills whenever the mood strikes him, just remove them from the game. The DM will decide when he wants your attempts at diplomacy or intimidation to work, so spend your skill points on more useful things. Some people just don't like social skills, and that's a valid point of view. If that's the way you want to play, formalize it so that everyone's on the same page. If the DM is letting players believe falsely that their ranks in social skills mean something, he's treating them unfairly.