Team Fortress 2.
I loved the original Team Fortress, which sets up the disappointment. If TF is a 9/10 game, I was expecting maybe 8 and up for TF2, and it ended up as a very mediocre, consolized 6 - most of the points brought in by well-designed graphics, amusing characters and technical prowess, not the interaction. Its very low cost as part of The Orange Box makes the lack of quality forgivable, but does not diminish it.
In TF2 the TF speed is dead - every class except scout runs slow as a snail. Switching weapons takes obscenely long. All this serves to bring down the skill ceiling and press good and bad players closer to each other. The payoff from out-thinking an enemy is diminished when you can't put the plan into action that fast. Not being able to evade at a high speed means many more unavoidable deaths (and many more free points to the nubs, which keeps them playing regardless of whether they ever get any better, which was obviously one of the core goals of the design).
The complexity of TF and hilarious gear like hallucination grenades and EMP grenades are gone. You can tell they went for the "HURR DURR" crowd that cannot handle >2 buttons. You can no longer pressure or bait people to use their limited-quantity weapons, or maximize the effect of an ambush on an oblivious enemy.
Flag-running might as well not be in TF2, it's totally butchered. Non-combat or relay runs won't happen anymore because the slowness of everything and the map design do not allow it. The good TF2 players have their matches in capture point maps, which is the new bread and butter.
Class balance is not there. This is not to say it was in the original - but I do expect and require it in a game made 11 years later. They completely destroyed the spy when they removed friendly fire, so there is no point in playing one against a skilled team, only against nubs. Same goes for engineer but for different reasons.
They put in relatively powerful melee weapons without setting up special mechanics for them. Huge blunder.
Then they made a retarded achievement-dependent gear system, only to replace it with an equally retarded time-based lotto. Inclusion of grinding is, as usual, the final admission of the developer that they could not, or would not bother to, make a game worth actually playing for long.