I'll just say it: pyros are OP. Thankfully, I like playing a pyro and unlike WoW, switching classes takes a few seconds, not days.
Here's the strange thing though: not everyone plays a pyro. Why not? There's still a rock-paper-scissors thing going on, and so eventually the rock to their scissors will be swimming in targets, making people want to play the rock, but then that makes paper pretty fun too, and so on, until despite being OP, pyros won't be totally ruining the game.
This of course requires a certain range of overpowered. They can't be so OP that other classes cease to counter them. For example, they still have awful range, so a class that can catch or keep them at range will have a much easier time than a class that has to fight them close in.
Which raises the question of what is so OP in the first place. Well despite being rock-paper-scissors, not every class is matched up exactly against another as a win or lose. For one, skill is supposed to matter, meaning that a class can beat its counter, it's just harder. So when a countered-class fight becomes a total pwnfest, something is wrong. Similarly, if the counter-class to the pyro has too much trouble killing it (for similar skill), then again, something may be wrong.
Of course map choice matters. Open maps are not friendly to them. They like sharp corners and enclosed spaces. Water is just awful.
Playing against them, pyros feel as if they have just a little bit too much health. Or they run slightly too fast, based on their ability to somehow always catch my scout, despite it being the fastest in the game. On the other hand, playing as one, they have massive damage output, but still feel slightly too durable, except against soldiers armed with the Black Box, which is OP (it heals slightly on each hit).
But maybe Valve ran their numbers and found that spies weren't getting slaughtered quite as fast as intended. So I'll play their game, literally, and figuratively, and go right on lighting people on fire. Because honestly, I just miss playing a demo warlock.