Big features include: Wiki-like tracking of edits, Post from wherever you are on the web, Turbo-speed your blogging (with Google Gears), Theme preview (like what has been available recently for WordPress.com blogs, and other things like word count, image captions, and the most important – a number of bug fixes.
Upgrading: 2.6 is pretty much identical to 2.5 from a plugin and theme compatibility point of view, so upgrades from 2.5 should be pretty painless. The 2.5 branch will no longer be maintain so everyone is encouraged to upgrade. Our standard 3-step upgrade instructions apply to this release. There were at least 1,984,047 downloads of the 2.5 series, the fastest growing release weâ€™ve ever had, and I think all of those people will find 2.6 adds a level of polish that really makes WP a pleasure to use every day.