In no particular order:
-The amazing art style, cheery, vibrant colors, light humor, AMAZING characterization (seriously, try describing each of the characters without saying how they look or what they do. You'd be amazed at how long you can go) despite very limited exposition/dialogue.
-Partly because of said art style (I don't mean to diss, but the people who find the "childish cartoon" vibe to be beneath them are usually teenage, immature pricks) the community is pretty relaxed and devoid of griefers. You get an unusual share of hackers and 1337 kids every free weekend, but for the most part players are pretty civilized and easy-going.
-The gameplay is surprisingly balanced considering the amount of stuff they keep adding in, and the strict limitations to each class's role and abilities really forces people to work together. There's also a much greater variety of guns, weapons and abilities, whereas most FPSes only feature hitscan weapons with varying degrees of power, accuracy and reloading speeds. Finally, gameplay is very focused on close range combat and is designed so even complete newcomers can contribute somehow.
-It's 20 bucks, for fuck's sake (and even less if you buy the Orange Box, which you totally should). TF2 is hands down the best value for my money I've ever played, AND THEY KEEP ADDING MORE STUFF FOR FREE.