Crabbe fouls too much.
From what I can see (and this applies to both Crabbe and Aminu, although Aminu is overall a better defender), there is a lot of activity when you're watching Crabbe. What I mean is that he's constantly moving, moving his hands, moving his feet etc. It makes you believe their good defenders because there's constant effort. I don't see it leading to results all the time. Henderson just plays solid, straight up defense, no ifs ands or buts. Sometimes he'll come up with a nice highlight block but for the most part he's just damn effective, no wasted motions. He just "knows" how to defend, if you get what I mean.
I'd say Crabbe is solid, but he's got a ways to go.