I'm no expert, but theorbittwo's assessment sounds about right to me. All in all there are some pretty weighty changes.

For example, check out this little gem from the xhtml FAQ...

Is <img> being replaced by <object> in XHTML2?

No. <img> is being replaced in XHTML2, but by something else ...

What XHTML2 does is say that all images are equivalent to some piece of content; it does this by allowing you to put a src attribute on any element at all. What this says is: if the image is available, and the browser can process it, use it, otherwise use the content of the element. For instance:...

Take a look at the elements in xhtml2 to get a sense of how much has changed.