Most of the changes in WordPress 2.0 are internal. However you will notice improved admin interface and some nifty features and some pain. I will highlight the key features for bloogers in this release.

AJAX goodies like ability to create categories while editing a post, more intuitive list management, fade-in/fade-out effects etc.

WYSIWYG editor with draggable sizing. I don't particularly like it as it is rather slow.

You can easily change the color of nasty-big-blue-blob on the default theme.

Role replaces user level. I am assuming tons of plugins which check the user level before allowing specific action will fail.

Posting with large ping lists should be faster. Last time they tried to add pinging to shutdown hook which obviously doesn't work. Haven't checked the code this time. Hopefully they have incrementally distributed the load to other admin activities only.

Database query caching to speed up access. Haven't noticed it on my blog. I am looking forward to see some numbers.

For once I am not going to be an early adopter. I will wait and watch and test on my test blog.