I just upgraded my blog to WordPress 2.7-beta1. I took the risk to use the beta release. The upgrade process run smoothly without any issue. The biggest changes are on the Dashboard interface. It took for few minutes to explore the brand new design. But, since I have started using WordPress from the Subversion, I have no problem with that.
There are som new settings and the more handy dasboard. Some personal highlights:

• Quick access links — expand the link groups. If you think the navigation eats too much space, you can easily move to the icon-style navigation.
• Hide/show menu elements — For example, when viewing list of comments, you can easily hide the details you don’t want to see e.g. comment posting date. There is a “Screen Option” link on the top navigation on some pages.
• Quick entry edit — If you only want to edit the title, post status, author, password, categories, tags, comment/trackback and timestamp, you do not need to edit using an entry screen. Just click on the “Quick Edit” link, and save. If you’re using Manageable plugin, you know about this feature. It’s built-in now.
• Threaded/nested comments — Settings are available from dasboard. You can have upto 5 levels of depth.
• Paged comments — you can now break comments into pages. Remember WordPress Paged Comments Plugin?
• Closing comments for older entries
• Plugin Browser/Installer — I wrote about this before.
• Date/time-related settings is easier to understand — You can select from the pre-defined values or create custom formats.
• Sticky post — If you like to have a sticky post on the front page, you can have this feature from the core. You can have this feature using WP-Sticky plugin.

Anyone using the beta version? If so, you should see more new features/improvements there… or probably some bugs.
I like it so far. And, for WordPress theme designer, it’s time to learn about new template tags and structure. Right now, I am still using WordPress theme built for 2.6.x, will modify it when I’m using the new features. Congratulation for WordPress developers!