#7: Friendly fire
There's a bug on your armor, hold still.
Anyone who has played a game online knows how annoying being shot by your teammates can be, especially when in the heat of battle. You die, only to find out it was your own teammate that put the killing blow into your cranium. Years ago with the Ninja Turtles arcade game, there were two modes: one with friendly fire on, so you could beat the snot out of your buddy as much as you wanted, and another mode that turns that aspect off.
It would be nice to see this as an option more often, but at the very least it's nice to play Gears of War 2: Horde Mode, and not have to worry about that friend that always runs in front of your shot.
#6: Technological Innovations
Look ma! No wires!
With the advent of wireless controllers, four player couch co-op is no longer a lesson in remember your boyscout training in untying knots that make no logical sense in how they got there. There's more room to spread out and less of a chance of being hit by someone when you accidentally pull a Leeeeroy Jenkings moment.
Rata-ta-ta-ta is the sound a keyboard makes, and it's one you'll no longer be distracted with thanks to voice technology. No online co-op session is complete without a headset, so connect to someone in a more personal way with voice chat using a wireless headset!
HDTVs allow split screen players to sit a safe distance from the viewing device - no longer are CRT beams being pulsated directly onto your retina as your four man squad has to sit 6 inches from a 13 inch television. Now with your 52 inch plasma, everyone has their own 13 inch screen to view!
#5: Music Games
Get used to this screen at parties.
Everybody wants to be a rock and roll star at some point in their life, and thanks to the advent of plastic instrument controllers, most of us can pretend to be. Gamers have become rockers in a band with instruments like Guitar, Bass, Vocals and Drums. Sure you make look like an oversized 7 year old with toy instruments, but damn it to hell if you don't feel cool.
There's simply nothing better when the band hits everything just right.