Three things will ultimately make or break IE in the long term...
1.) Interoperability that isn't double-faced, e.g. open source codec support out-of-the-box for the audio and video elements.
2.) Standards compliance that is at least reasonably on-par with other browsers.
3.) A GUI that is usable, IE9 is less customizable than Safari for crying out loud!
Those are the changes that IE10 needs to be successful and I think Microsoft will have a lot of trouble with points 1 and 3. I do think their standards compliance approach is pretty solid at least by implementing more mature specifications they're less likely to have to fix stuff in IE10 that changed since implemented in IE9.