I'm not much of a photographer, but...
I agree the composition is pretty nice, as well as the lighting, though they could benefit from a dark setting.
The biggest thing I find is that they seem boring/not exciting to me. They don't really seem to capture the movement or difficulty (I never got into aggressive, but I'll assume this stuff isn't super easy). To me, it almost looks like some guy just posing on the coping. I just feel like there should somehow be more tension, and perhaps something to draw the focus to the subject a little more (it's probably the building in the background that is drawing my eye away). Given that you couldn't shoot at dusk, maybe having the person take up a little more of the frame, or capture them in a less crouched position(?).
Also, I think they could benefit from slightly more saturation.
Like I said, I don't really know what I'm talking about, so it's possible I didn't describe what I'm thinking in the appropriate language. You also might chose to disregard what I have said. I do, however, think that as someone without a ton of education/experience relevant to photography, that I can offer some insight into what a more general audience might see, which can be beneficial, considering the majority of people viewing your work know dick all, and will not appreciate a lot of the more subtle or complex things that an expert might comment on (at least they won't explicitly notice them, though they will likely have a positive impact on there overall appreciation).
Hope this makes sense.