From the essential “Theory of Fun” approach, CC’s combat gives your brain plenty to munch on. In short: there’s absolutely no way a game like Double Dragon or Golden Axe would do well with today’s gamers. Castle Crashers has developed a far deeper combat system, that puts it closer perhaps to Devil May Cry (especially in being juggle-centric) than the older titles that ostensibly inspired it.
Of course, a major draw of Castle Crashers is the co-op play. I don’t even know if I would have purchased CC without online co-op. I can’t think of a single other time that’s been true: co-op is often treated as an early, easy feature cut, and for good reason. Does that equation change for XBLA titles? You know your players are online. But plenty of other titles have assumed their players are online– it’s just usually used for competitive play. (In my post title, I have a bit of fun with qualifiers. MMOs are co-op successes. Golden Axe was a co-op success. Still, I find Castle Crashers to be a surprising outlier, given today’s console game environment.)
On a side note, while everyone accepts that people enjoy playing games together, I haven’t heard an analysis of this to nearly the extent that Theory of Fun tackles mechanics. This is probably well-trodden territory by psychologists, but how many developers read psych journals? It sounds like the “massively solo” success of WoW caught even its creators by surprise.