What exactly are people talking about when they say that a game is just a "Power Fantasy"?
That will depend a lot on who you ask.
For me the term implies that a game is about empowering the player character so that they a free from responsibilites, restraints and risks. The player doesn't have to worry about powerful enemies because they know that with persistance they can all be defeated. The player doesn't have to worry about their boss taking away their badge and gun because they know they are always right.
That's problematic because constraints are interesting. Fighting hunters in Halo is awesome because they will kill you in a moment's lapse of concentration. XCOM's campaign is very interesting because you can lose. However, I think it's a sliding scale. Most games are power fantasies at some level, it's a question of how much and what problems arise from it.
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Geralt is interesting in that he defies everything that makes his world so grimdark.
The world is full of horrible monsters, yet he kills monsters for a living. That pretty much makes him the apex predator right at the start.
He's a supernaturally good swordsman, making him fearless of all the human thugs running around robbing people and raping beautiful peaseant girls.
His mercenary like job makes him independent of the horribly corrput feudal governments oppressing everyone around him.
He's immune to disease, so he doesn't have to worry about the constant sickness getting about the squalour ridden areas, also free to have sex with anyone.
He has magic powers, because life wasn't easy enough?
I like the character and think they manage to tell an interesting story about him, but there is a lot of power fantasy there.
(edit) However it's worth noting that he can quite spectacularly screw up through plot choices, at least in the first game.