I love how simple and straightforward the UX is. It was instantly obvious how to use it, and it worked quickly and perfectly without any modals/banners/ads/whatever.
I think a single sentence somewhere on the page after you stop the recording telling you that you can crop by clicking and dragging, and you can trim by moving the little time markers might help with discovery.
I didn't realize you could do either of those things until I read your comment here.