Ah, Counterstrike... that stuff is pure video crack. I haven't played in over a year, but I still think Counterstrike best incorporates several elements that make people want to play games. Game developers would do well to incorporate these into all their designs.
Cooperative play: It's always better when you can work on a team and have people backing you up, then simply "shoot everyone deathmatch". It creates a (yes, false) sense of belonging and camaraderie.
Sociality: People tend to pick one server and play on it consistently, so you see and get familiar with a particular group of players. In the couple of minutes between when you're unlucky enough to get killed and when the next round starts, there's a lot of chatting going on.
Short rounds, rapid rewards: Rounds in CS are short, no longer than five minutes, and usually much shorter. If you are winning, you win quickly. If you lose, you don't have to suffer much. You feel that don't have a whole lot of time invested in the game either way, so winning or losing one round isn't a big deal. No matter what happens in this round, there's always another round coming right up. But of course you don't realize that you are actually blowing a huge amount of time on the game, because you keep telling yourself, "okay, I'll play just ONE MORE ROUND..."
Multiple rewards: You can play the game several ways, depending on your style. Want to plant a bomb? Do it and get some money. Want to defuse the bomb? Do it. Want to win by wiping out the other team? That's your thing, man, wipe away. Even if your team loses the round, you can still personally score big points by getting some kills. It makes players feel that they are contributing in their own way.
Think about some of the games, especially online games, that you spend lots of time on. Chances are, they'll include the above elements in one form or another. Battlefield 1942 has done a really good job of incorporating these elements as well, which is why it's now so popular, but Counterstrike so far has done it best.
