Sounds like you want a game with no dm at all, then. If the dm doesn't make decisions, what, exactly, is his job? To memorize page numbers?
I agree with [MENTION=27160]Balesir[/MENTION]'s reply to this.
And as I said in reply to someone else's similar comment quite a bit upthread, the notion of "making decisions" is underanalysed. Here are a range of decidions that have to be made in a typical RPG session:
* Decisions about backstory for PCs;
* Decisions about backstory for PCs' friends and family;
* Decisions about backstory for the rest of the gameworld;
* Decisions about what is happening here and now - who meets whom, and what mood they are in when they meet;
* Decisions about actions taken by PCs, and the outcomes of those actions;
* Decisions about actions taken by NPCs, and the outcomes of those actions.
Balesir is talking about the interaction between the last two categories of decision. Even if, in certain circumstances at least, a player has more authority in respect of those decisions than the GM, it hardly follows that there is nothing else for the GM to be deciding. Apart from anything else, there might be another, uncharmed NPC present here and now.
It requires no more dm fiat than a dm ruling whether or not it's cloudy enough for call lightning to work, or if yelling loudly will waken a sleeping companion.
I'm 99% confident that these would also fall into Balesir's "dislke of fiat" basket. They certainly do for me.
When I GM (and when I play) I want the players' choices to determine whether or not their PCs succeed at dealing with the challenges that confront them. Every time I, as GM, have to make a decision about how effective the players' actions are, I am undermining the contribution made by their choices and making the outcome a matter of my choices.
So how do you work (or house-rule) diplomacy or reaction checks where the descriptors are similar to the old charm person? (If that's not "direct influence" then why does the charm person need to be?)
Balesir can answer this for his own case. In my case, via "intent and task": the players indicate what they are hoping to achieve via having their PCs engage with the NPC, then checks are made, and if the players succceed they achieve their intent.
Is the difference between this and the GM setting the DCs for challenges (such as deciding what the NPCs will save is) where the GM decision comes in to play?
The difference is between posing a challenge and dealing with it. The GM's adjudication of the charm spell doesn't pose a challenge that the playes respond to by deploying further resources. (This contrasts with, say, a skill challenge, which has exactly that character - the players don't realise their intent until the challenge is successfully completed, and in the meantime they
are deploying further resources to deal with the unfolding challenges of the GM's ongoing narration of outcomes of individual checks.)