It's fun and very accessible to a wide variety of gamers without being cliche and tired. It also gets updated often and has a great deal of player-made content which attaches the playerbase to the game even more.
Brony servers. Trading and hats keep casuals interested, plus a bad player doesn't stand out because there are so many of them. At the same time it is possible for an above average player to carry his team (see: every Star commentary ever) and for the nolifers to play competitive.
As fundamentally far as I am concerned, a lot of the metagame has separated off anyone with real interest in getting into a game. Too hard to catch up now. Still free to play and fun for the casual at different levels. Initially, it was on the Orange Box, was not very contested in it's market (class based team shooter), and had very entertaining fresh aspects. Things with serious-toned cartoony humor, usually owes it's roots to it, including MNC. Class based shooters usually lean to how TF2 was done now, including MNC, when before TF2 other class based games usually rotated around different things entirely, like C&C Renegade. As it received it's first year of free updates, it simply kept it's playerbase while getting new players, which was amazing and unheard of at the time, a game receiving free content all the time, so it is no wonder the old players stayed as they got to play a different flavor every month. And that is why TF2 is popular. I played vanilla on Xbox for 2 years, but I didn't need content patches to be a dedicated player of anything that was entertaining on a competitive level. Back when I played it on Xbox, I didn't play anything beyond Runescape on computer, on laptop, while I played CoD and TF2.
