HTML 5 adds new features that will benefit both web sites and web applications.
While the standard still is work in progress, modern browser already implements a large number of the features, and new features are being added at a breakneck speed.
Any browser that want to remain relevant in the future needs to implement HTML 5. This means that it is time for us developers to stop wasting much energy (and our customers money) on trying to bridge the gap to the faulty browsers of the old days (read Internet Explorer). The gap is simply to big now, so it is no longer feasible - instead the users should upgrade to a moderne browser (or alternatively install the Google Chrome Frame)
HTML 5 includes features that will benefit web sites in general. It is possible to do a better markup of the contents which in turn will help search engines better understand what is on the page.
When it comes to web applications HTML 5 has even more to offer. It is now possible to build desktop-like applications without having to resort to clunky browser plugins.