Well clearly 360 and PS3 version, we need to say no more.
PC version, VALVe show no sign of slowing down the number of updates, particularly with the new community content. The art style is timeless, and then underlying graphics, (i.e lighting and such) are still acceptable, so no problems there. A huge playerbase the seems to still be growing. One advantage is it's now so cheap, (extra so on sales), and easy to play on old PC's, that ALL my friends have it, 1/3 have MW2, 1/2 have BFBC2, but if we want a big game, TF2 is the place to go. It has few real competitors, MW2 is a very different type of FPS, lone wolf based, BFBC2 has very different teamwork.
IT WILL GO OUT OF DATE
Let's make that clear, but VALVe will step in and make TF3 or an equivalent to fill the gap before we need to worry.