That I am too poor to pay for it when not working.
In all seriousness, I think it's a great way to get motivated and excited to work out, but I've also seen many (albeit mostly old) people with scary bad form in a workout. Supposedly the on-ramp classes are supposed to help introduce most of the movements you use, but it's really easy to miss someone in a large group workout.
That said, it seems most of the Crossfits I've visited are pretty good at recognizing when someone is having large difficulty with a movement and providing alternate exercises or dramatically decreasing their weight. If there are a few in your area, check them all out. And find out if they run a particular weekly schedule, ie, if particular days are usually weight days, gymnastic movement days, or named workout days.
The biggest thing I think you would need to worry about (for your knees) would be box jumps, but those are easily scaled. Other things to consider are the high volumes of jumping rope, rowing, KB swings, and squats, but if you can cycle for an hour, those might be less worrisome at low weights.
(sidenote: I'm having difficulty deciding which crossfit to sign up at. The one <1mi from my house has a coach who I formally had as a gym teacher in high school because he was fired for inappropriate conduct with students. The other one 15 minutes away is much smaller and the coach I encountered was sort of young and douchey.)
