I'm saying that once the GM is obliged to be open about what s/he is doing - for instance, that such-and-such a thing is or isn't permissible in the fiction - this opens a space for negotiation between the players and the GM. Which is empowering.I do see how this is transparent (like objective DCs), but I don't see it as empowering players. With objective DCs, skill list uses, powers, action points, etc., all the tools are in the hands of the players. That empowers them to act. That's why people in this thread have said they love powers and the power system.
On the other hand, while saying "I don't think you're powerful enough to do this" is transparent, it doesn't empower the players, does it? Or are you saying that once the PCs decipher the GMs (hopefully) transparent reasoning on allowing some things and not allowing them, they'll be able to declare more actions than they would before figuring it out (and thus gain more power)? I could see that argument, but my gut doesn't tell me it trumps direct player empowerment ("you, as a player, have the power to do X.").
I guess it depends on the skill. I don't think any of my PCs can hit a Hard DC in something they aren't trained in, for example.
Whereas if the GM is simply pointing to an "objective" DC 50 that "makes sense", the player has no opening in which to express a contrary view of the fiction, and to engage the GM in discussion. (Or if s/he does try to do so, because of the terms in which the negotiations will be framed s/he is likely to be labelled a munchkin or a rules lawyer.)
Furthermore, in practical terms, once the GM is overtly negotiating permissibility within the fiction, and everyone knows that if the particular possibility is OKed (eg using Twist of Space to free someone from a trapping mirror) then there are a clear set of mechanics/DCs for handling it, the GM has no particularly strong incentive not to OK it. Or, if the GM's real concerns are not the colour/tone of the fiction ("Too gonzo!") but rather the plot implications, then it becomes much easier just to be upfront about that.
So the empowerment, at least as I have experienced it, lies in the change in the social dynamic that results from transparency about what the nature is of the decision that the GM is making.
On the issue of DCs: a Hard DC at level 1 is 18 (isn't it?) which is doable though hard for an untrained PC (the chance drops down to 10% for an untrained 8-stat PC). At higher levels the gaps grow but so do the opportunities for buffs etc.
This does give players an incentive to push for solutions in their areas of PC competence. I've seen this derisively described as "roll your best skill and make up some nonsense". For me, it's more about conceiving of situations in ways that speak to the personae of the PCs. The player of an 8 CHA, athletic PC will always be pushing for situations to be physical in their character and resolution (somewhat like REH's Conan). That's another form of empowerment, as a form of co-authorship.
Of course the GM can block. But this goes back to my point above: the GM's blocking is, overtly, a block around tone/genre rather than ingame considerations, and so has to be owned and defended on those terms. If the GM allows it -and why not, if the player wants to play a Conan-esque athletic character - then there won't be a mechanical block, but rather the standard chance of success or failure and narration of consequences either way.