The document type has been simplified. Before it used to have horrible things like
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"
But now it has been simplified overmuch to just
<!DOCTYPE html>There are some people who like this (e.g. John Resig). But I don't. HTML will continue to evolve - there will be new tags and attributes, and the existing behaviour will be clarified or changed. But without a version number, how will a browser (or any user agent) be able to work out which version it is dealing with? And how can a content generator signal which version it is creating? Already there is considerable confusion about which bits of HTML 5 are supported by different browsers.
The simple answer is perhaps that this allows vendors free reign to do what they want - and we saw what a mess that caused before HTML 4 put a standard in the ground. There is still time for the WWW Consortium to fix at least this one error before it is too late.