Yea, this is what digg didn't have - established micro-communities. I can't imagine /r/blender, /r/learnprogramming or other such subs to ever feel the need to move unless something much worse happens. Or, of course, an alternative providing such a significantly better experience than the existing option, but as the main influence on that factor is the level of activity in a community, it's hard to imagine how that would happen.