This guy has been "working on" Spy Party for like 4 years now. What's taking him so long?

Totally Spy Party.

Maybe the fact that it's one guy?

You can sign up to the beta for $15 if you're that eager to play it.

It's a small team, I believe only one person is doing all the heavy work. It's also a seemingly complicated game. So be patient, because games take forever to make. Think of the guy who made Dust: An Elysian Tale, even though it was his first game, it took him like 3-4 years to complete that game all by himself. Fez took forever as well.

Chris Hecker, the developer of Spy Party, recently tweeted that the game is about two years from completion. So, 2015 is the best guess. As to what is taking him so long, it's really only one guy working on it as I said.