Surprisingly, this upgrade wasn't intended to add a few more tags or incorporate a couple of CSS extensions into the language. However, you will notice some differences. The first has to do with the fact that this is an XML document. One group wanted to define only one DTD for the specification, arguing that it would help keep the language simple. The core HTML within the document followed clearly delineated constraints, and any non-HTML code added to it would need to be added under a separate namespace.
Frames are for the most part independent of the content that they contain.
Namespaces have become fairly common in XML circles lately, but if you're working in a strictly HTML environment, chances are you've not encountered them before. The default namespace is one where the tags don't require a prefix to identify them. This has benefits for both creating sophisticated server-side code for displaying such information, as well as for creating modular output that contains subsets of HTML for different platforms.