Anne van Kesteren har written a great article at W3C about what differences we might expect between HTML4 and the new HTML5. The read is easy to adapt and Anne explains in plain text what elements are new and how they differ from previous versions.
Besides all the new semantic elements that will be introduced, such as
<footer>, there are some other interesting new things to be prepared for:
lielement that today can contain both inline and block.level content thanks to it’s flow definition in the doctype, will only be allowed to contain one of them in the future (but you can still nest inline elements inside a block).
ielement will not be deprecated, but instead represent span of text in an alternate voice or mood, or otherwise offset from the normal prose. It will still be italicized and will probably still be misused by authors.
- The Doctype will no longer require a reference to a DTD.
<!DOCTYPE html>is enough.
Ther are a lot more to digest in the post so make sure you read it.