Sorry I should have been more clear in the question:
:: User Experience (This also could subjective in a broader sense - but what I mean is the usability or easy of use)
:: Browser rendering (Clarity and quality of the rendering)
:: Easy implemenation
:: Format support
The lion's share of visits are increasingly coming from mobile platforms, where Flash either isn't supported or works poorly. Even Adobe has given up hope on making Flash a viable future platform for video in the mobile space.