On the other hand MediaWiki is overly focused on Wikipedia. Without Wikipedia, I think that the default MediaWiki would look a lot more like Wikia than like Wikipedia. In my opinion, if MediaWiki treats Wikipedia as it's sandbox then it does so because the only sandbox that compares to Wikipedia is Wikipedia itself. There ARE no other viable experimentation grounds that compare to the distorted reality of Wikipedia.
So how bleeding edge is bleeding edge? Code is deployed almost every 2 weeks, yet HTML5 has been the default for MediaWiki for over 3 years now, but has still not made it to Wikipedia for all sorts of compatibility reasons and accommodating to the volunteer tech community.
HTML5 mode is currently scheduled to be deployed this summer.