From the start of videogaming, players have been pitted against countless numbers of enemies, of countless types. The list of different enemies is far too extensive to list, however some are incredibly inventive, others are used far
more than many think should be acceptable (zombies, aliens, and Nazis being three of the biggest culprits). However, from the small writhing pods of the Flood, to the massive Colossi, I don't think any other enemy has been such a staple in the arena of video game advesaries as one group: other people.
Think about it. Even from the humble roots of gaming, such as Pong, one adversary has nearly always remained a constant figure to be reckoned with...at least usually. Both locally and across the world, gamers love being pitting against each other, possibly even more so than being pitted against computer-controlled opponents. Even today, games such as Call of Duty 4 and Halo 3 have immense online multiplayer followings, where players spend countless hours playing against other gamers
Just think: one of the most satisfying feelings in a game is fighting against someone who is your equal; a battle against someone who poses a real challenge for reasons other than having strength far greater than your own, or too much health. An enemy who can match you move for move, that actually requires skill and thought to defeat; An enemy that can see your trends and adapt to play more sucessfully against you. Of course, this group varies from the extremely skilled, fair players, that enjoy playing the game, to the vulgarity spewing, bigoted, enfuriated douchebags who think tea-bagging is the funniest thing in the world.
^ These guys
Both online and offline, there's something so enjoyable about playing against another actual person. Part of it seems to be that, like previously mentioned, skill is a part of it, but generally speaking, you're playing against someone that's equal to you, an enemy capable of adapting and changing more than any current AI can, that provides an honest challenge for all the right reasons. Another part of it seems to be that you're playing against someone that is having the same experience you are, so in a sense you're sharing the experience with them, even though your bullets are aimed at them. I think, to a degree, a sense of community among gamers is one of the best parts about gaming, and playing against other gamers strengthens the community, by allowing you to share the experience, and by allowing you to form alliances in the forms of clans, and forming deep pitted rivalries.
All in all, the basic idea of this is that the best enemy that gaming has ever put players against, is actually other gamers. And I think the trend of having players play against each other is one trend that's going to stay around, for all the right reasons.