from a practical view, 2.7 changes the way the admin sees wordpress (which i really like). the admin interface is clean, much more web2.0 and easy to navigate. i have not tried themes or plugins, but the system itself seems really clean, easy to integrate and when 2.7 appears for the masses- if wordpress holds true the update/upgrade will be easy.
in my test i installed 2.6.2 from godaddy. i then removed the entire wordpress folder and files (leaving the database) and copied over the 2.7b2 files. i changed the config file to match the file that was originally created when godaddy generated the database and uploaded that.
opened the 2.7 index page, the system told me to update the database (which i did and with one post took 15 seconds) and off i went. Logged into the admin and all was right with the world.
if all holds true, the system will show an update to 2.7 “soon”, i will upgrade to the version, update the database and off i will go! anyone else trying 2.7, let us know!