TF2's cool when it's all released.
When they have tied the achievements with the weapons, this is where new problems started. On every release of new weapons, people would grind and grind on getting the number of acheivements so that they acquire the weapon. Needless to say, this wrecks the gameplay, and teamwork element is all gone by then.
Luckily they have added dropouts and trading to counter such problem. The grinding has been significantly less ever since, but they have "idling" servers just for getting the dropouts in a duration of the game while doing something else. That doesn't hurt other gamers because these "idling" are usually done at the other servers.
When it's F2P - newer problems are introduced. In my nation, sadly speaking, many of the new players are playing in a manner of Dota, or L4D. When these people group together, it will be like Beavis and Butthead
playing and fighting together. All the teamwork element are gone, only leaving players beating other opposing forces for the sake of "high scores" or because they looked like zombies. When they kept dying, they would just leave and said "this sucks".