So I just played the demo of Castle Crashers and although it was all right it just reminded me why I don't play these sorts of games. I can't keep myself playing when it's more or less the same thing for every level. Kill some dudes by hitting a couple of buttons repeatedly. This is the same reason I generally don't like Zombie shooters. Not only is it painfully repetitive but zombies may, quite possibly, be the most rubbish video game enemy ever conceived. For the most part they're usually slow and bumbling about and you can take them out before they get close to you or they're of the variety that just run and jump at you in some suicidal attempt to be mildly irritating.

Repetitive gameplay isn't a problem on it's own. It becomes a problem when the developers seemingly do little or nothing to counter balance it. Forcing the player to think about what they're doing, make strategic decisions is one way to distract from the repetitiveness. Another solution is to create interesting, unique scenarios around the core gameplay. Put the players in positions where they are doing things other than whatever your core gameplay happens to be (Shooting dudes, smashing dudes, slicing dudes... killing dudes in general). Most FPS's do a fairly good job of this giving you specific objectives to complete (Y'know other than shooting dudes, smashing dudes, slicing dudes... killing dudes in general).

I don't have a real conclusion to this so... the end!