My only real gripe about JSX is the crazy attribute casing rules. If it looks like HTML then attributes should be case-insensitive just like HTML. The worst of it is (unless this has changed) they are just silently dropped without any error message.
That's a good stop-gap, but the tool itself should either pass through everything to the HTML or throw errors on anything that isn't going to get passed through. The silent failure mode is a UX bug.
