I think another common one is that the inner quad doesn't develop as well as the outer quad, because you don't straighten your leg out fully when cycling. That can lead to funky knee problems. You can do exercises like standing on one foot, bend the standing leg as far as you can balance, then straighten it all the way, focusing on flexing that inner quad at the top.

Or ... you can do lots of different stuff =) I don't ride my mountain bike as much as I used to, but in a typical week I will ride either my road or mountain bike, play soccer, and climb at the gym. I seem to have fewer joint problems than when I was just riding. I don't stretch enough, though, and I'm sure that'll come around to bite me. Oh, which brings up another great cross-training activity, yoga. Yoga is great stuff; unfortunately, I tend to avoid it because a lot of the positions involve putting weight on your palms, which is a problem for my bad wrist.