1. There is no secure media streaming in HTML 5.
2. No good development tools to build animations and interactive content. Developers need to start using java script and CSS for the above activities
3. Browsers do not seem to have full implementation to support HTML 5. The popular old version of browsers do not have HTML 5 support too.
4. Each Browser supporting HTML 5 has its own rendering approach and there is no common thing in all of them to replace Flash. Developers need to write separate code for content rendering for each browser.