TF2 is the perfect online multiplayer experience because it embodies what I call the "weekend warrior" philosphy.

No commitments, no time gated nonsense, no bullshit. Pick up and play as much as you want, as long as you want.
Large teams with simple objectives encourage loose teamwork, everyone can contribute no matter how they play.
Relatively straightforward but distinct classes to play, no roster bloat or having to worry about a class being outclassed in three months time due to a new release.
Casual, community and competitive (lol) servers can be found. There's always a little bit of something that'll suit you fancy.

TF2 is *the* game for weekend warriors. Plug and play, frag out and have a good time. This is what online multiplayer should be.