The "import repository" page does not make it clear that only the latest revision will be imported. This is frankly a little uninteresting.
It would be much more interesting to use hgsubversion to do the conversion. Bitbucket has a chance to add a much-needed graphical interface to it with nice input fields for the various things that can be configured with that extension. People also sometimes struggle with installing the Python bindings for Subversion and an online service would help here too.
Making a "proper" import like this would of course be much more demanding in terms of both CPU time and disk space, but I think you should consider it.