I took the plunge last night and began switching a live site to HTML5. The task was not all that daunting, but it did make me eat some of my words. Just because a site is valid HTML or XHTML, does not necessarily make it valid HTML5. It is because of the obsolete features that this was the case for me. Once I got them in the bag, everything was ship shape, at least for this phase. The site segment that I revamped is still largely old code that needs to be thought through and tagged with the new semantic tags. That should be fun.
Here is a list of the Obsolete items as described on W3C:
W3C: Obsolete features.
Scroll down to table and get a gander at that section. Tables are taking on a stricter role, and layout is not in the mix; ergo, the summary="" empty attribute is obsolete.
What other validation issues are folks running into?