If I absolutely had to?
Easy: Guns. Getting rid of guns would make for the least radical change in the way games work.
Multiplayer is irreplaceable. Sure, singleplayer games are great too, but to remove multiplayer completely would remove a huge segment of gaming.
Magic is a very broad concept. Sure, it can be replaced with sufficiently advanced technology, but magic is a very useful tool that can be used in a lot of ways. Sometimes this makes for lazy storytelling ("it's magic, we don't have to explain it"), but it opens up so many possibilities that it'd be a waste to get rid of it.
Guns, however, are something very narrow. They're one specific type of ranged weapon. They can easily be replaced with all kinds of things, including magic. I can't think of a single game that wouldn't work if you replaced guns with, I dunno, magic crossbows or something. Sure, some settings (like most 'realistic' modern combat settings) would be very weird without guns, but mechanics-wise nothing would have to change.
I honestly cannot imagine why multiplayer is winning the poll (so far). Is the stereotype true that most gamers are lonely geeks who fear interaction with others? IMO, there are no gaming-related things that are more fun than playing a multiplayer game with some friends.
On the other hand, without Multiplayer you would lose a lot of a games charm. Like L4D and Call of Duty MW2
