I'm pretty sure it will be generally different for XenForo 2.0. Mike has already talked about the inclusion of LESS with the release which gives them the opportunity to run through all of the CSS and adjust accordingly. XenForo has a 2010 web feel to it and I doubt they would let that type of opportunity pass by to give it a 2015 feel.
It's likely just going to be different colors, enough so it's noticeable and feels like a new style, but still within the XenForo "realm," if that makes sense. For example, the message user info box could be a darker blue in XenForo 2.0.
Personally I prefer the current theme. I actually use the "Kitchen Sink" version on here.
Really though, the default theme doesn't matter as much. The important thing is the building blocks provided. Then sites can go on and customize it to make it something special for their own communities. Big forums like www.avforums.com, www.macrumors.com, etc have done this and their sites look great. I think after 5 years, no matter what the default theme looks like, you will think it's dated or old and needs to be redone. It has nothing to do with it actually being dated though. The default theme is far from dated. Sure it uses some stuff that isn't really standard at the moment (rounded corners, kind of gradient heavy [not really], etc) but it's still coded in a very modern way.