All of your Achievements count in Horde. There aren't any specific unlockables in Horde, but there are unlockables in the main game. If you encounter certain enemies in the single-player campaign, you'll unlock them for multiplayer play, such as the Locust priests.

How important is a co-op experience these days for a shooter?

In the last few years, it's gone from being 'nice' to being 'necessary.' It was a feature that gets planned early on in a lot of games but later gets cut. In a post-Halo, post-Gears world, I think you've got to have it. It's a great way to expose an inexperienced player to a game, and allow them to have fun. It's great for getting girlfriends to play, too.

What games do co-op well, in your opinion? Who are the guiding lights of co-op?

Not many, to be honest. Most people play Halo co-op, and Halo is one that does a great job of it. One thing I hate in co-op games is this [gestures in an "alley-oop" position]. That needs to go away. Of all the things you can do with weaponry and moves in co-op, your big unique idea is to give somebody a leg up? Really?!