Obviously I can't exactly force you to appreciate the TF goodness, but reasons I like it include:
a) It looks amazing, in that cartoony way.
b) It's funny as fook, particularly when you have a look through some of the archive material (Google "Team Fortess - Meet The Team"). And show me another game where you can throw a jar of piss at someone and then whack them with a kukri.
c) It doesn't take itself too seriously, and neither does its player base. When you compare it to the likes of COD, you don't get anywhere near so much bitching or comparisons of e-Peni.
d) You don't tend to get many cheaters - in fact I can't remember ever seeing one. I've had COD a month and I've already seen four confirmed aimbots.
e) Map sizes and variation make for some impressive matches.
f) It runs like a dream on practically any PC made in the last four years.