...says the guy insisting it's okay for a DM to force a player to set aside their definition of fun.
That's a nice cherry-pick. You omitted "any more than another player".
Which is pretty typical of how these conversations go: The DM is left as a servant and the player gets to make demands because otherwise the DM is ruining their fun, getting on an ego trip, or not thinking of others. Meanwhile, the guy coming to the table demanding that he
must be able to play exactly what he wants is only asking for what he's due.
In reality, that attitude is as petulant as demanding to play red in Risk and, when the other guy who really wanted to play red offers to dice for it, you throw a fit because he's not being fair. That sort of player doesn't deserve a seat at the game. I'm part of a monthly(ish) "board game night" where the games are discussed and picked ahead of time. The host sets it up, with the playing pieces all counted out and ready to go by the time everyone gets there. If someone pitched a fit because the red pieces they wanted to use were set up so that they had to read the map upside down, they'd be booted. That behavior isn't just inappropriate for a game, it's inappropriate for life.
When I GM, the game doesn't have to fit my vision precisely, but I do have to be able to put together a plan that feels right. There's a lot of leeway I'm willing to give to try to make sure the players all have fun. But... I'm playing the game to enjoy the game, myself, and I'm only going to go so far before it starts to feel like the player is being a bad sport. Sometimes, it's not even the specific request; it's when the request becomes a demand. If you're a jerk, you get no rights.
Sometimes, a character just rubs a GM the wrong way. I've stopped watching TV shows because a character was introduced that just sucked (IMO). That doesn't mean the character is objectively bad. I
do think it's a bad plan for all involved if I'm cheering for your character to die, though. It's much better to be up front that I'm done with drow forever and I'm probably going to subconsciously (i.e. without intent) avoid any plot lines around your character, transfer my irritation to NPCs, and generally treat you as supporting cast.
GMs don't get to go on an ego trip, but players don't have an entitlement, either. I really have no way of knowing the experience of anyone on this thread. Maybe they're saying "GMs should let the players choose what they want" because they've had GMs that used the game as a way to tell
their story, without regard for the others at the table. Or, maybe, they're socially maladjusted with a heavy sense of entitlement. Maybe the "GMs get to restrict your choices" advocate has had players who've dominated the game with a mary-sue to the point where the other players all dropped out. Or, they might be the bully, themselves.
Really, the only thing that makes sense to me is that "it depends". Sometimes a concept just doesn't work without a lot of work or it throws the flow of the game off (which creates a lot of work). Sometimes it adds huge value to everyone at the table. If everyone is a grown up, it shouldn't be a big deal. Talk it out. I don't think it's unreasonable to say that you don't get to bring a klingon with a disruptor into a Dark Sun game, but a mul wizard shouldn't be a problem, even if it's very odd (the second, third, or forth mul wizard might be harder to integrate, though).