Incredible writing, humor, world, story, characters, everything. The game is fantastic too. Valve mostly abandoned TF2 but it's astonishing that it lived (and continues to scrape by) for so long; it's a testament to how good it is.
I've given up hope for this game but at least it was fun back when I played in in middle school (except for the times my computer freaked out while playing this).
One of the best games for anyone who played it at the time, and this was the first game I spent money on in my life, I wouldn't recommend playing it nowadays, but it was remarkable, it was.