I can think of plenty of nit picks in TF2
1. The economy (the price of items and hats) is completely bullshit, due to many reasons: The rarity of hats, the low price of certain items, the fact that users decide which price is preferable, and most importantly, fucking everyone is trying to rip you off and make a profit.
2. The respawn times screw you over and are usually too long
3. Too many Snipers/Spies
4. Valve hasn't released any new five capture point maps, and are always releasing Payload and Payload Race maps.
5. Valve needs to look up the word "microtransactions" in a dictionary. The new Grodhart laser pack is $25 for four items. The Uber Update costed a whopping $100. Certain hats alone can cost $20.
6. Heavies are overpowered in the fact that Valve seems determined to get rid of all of its cons.
Heavy's too slow? Gloves of Running Urgently
Heavy's too weak with no Medic? Dalokah Bar/Sandvich
Heavy's too weak against Scouts and Pyros? Natasha
Heavy's too weak to Snipers? Fists of Steel
Heavy's too loud, and the spin-up time is too slow? Tomislav
Heavy's not good enough in melee combat? Warrior's Spirit, KGB and Buffalo Steak Sandvich
Heavy's not powerful enough (who would really think that)? Brass Beast