That's what the Gears of War 2 Horde mode does. In fact, Epic could have just released this as a standalone game called Gears of War: The Hordening, and we'd still have picked it up. It trumps the regular multiplayer modes – broken matchmaking complaints aside – because you know that no matter how great the match is, or how skilled your teammates are, it'll all be over in a few rounds. Horde mode gives you fifty (50!) waves of enemies who grow more skilled and more difficult to down as you play. That's pure adrenaline-fueled addictive gamecrack right there.
Sure the campaign co-op mode is back, and the campaign now gives you the opportunity to ride on top of giant vehicles, drive tanks, and eventually pilot a Brumak – like Cliffy said (a few dozen too many times): Gears 2 is bigger, better, and more bad-ass. But it's easy to underestimate just how addictive Horde mode can be. If they ever created a DLC Horde level where you're piloting a Brumak, that's just about pure gold waiting to happen.
If all that isn't enough, Epic has sprinkled several upgrades and tweaks throughout this sequeltastic experience. If you hated that the old cover system that had you sticking to walls like Colorforms in '82, don't worry. That's been fixed. Now you don't have to peel yourself off of a concrete block to avoid getting killed. New weapons? Indeedily do. Nothing says "Howdy neighbor!" like the cranking barrels of the Mulcher, which is just one of several new ways to turn your opponents into ground hamburger.
From chainsaw duels to meatshields, Gears of War 2 offers an arsenal of improvements over the original (which, for the curious amongst you, was Joystiq's 2006 Game of the Year), along with plenty of colorful language and tons of gore -- including a lot of new choice quotes from The Cole Train. What more can you really ask for?
Next: For people who drink and get high a lot.