As I noted before, I have begun development on my first Drupal 6 site.  I must say that so far, really enjoy the new system.  Here are a few things I have noticed:

• User access page improvement: One thing that has annoyed me about every Drupal 5 site that I have built is the user permissions administration page.  The page is a very long list of checkboxes which gives granular control of what different users can and can not do on your site.  The annoyance was that the user role names were at the top of the page.  However, by the time that you scroll down most of the page, the role names were no longer visible.  So you would have to remember which role was the first column, which was the second, etc.  This has been fixed in Drupal 6.  Once you scroll down past the role names, these names "float" at the top of the screen, making it obvious which column corresponds to which role.
• Drag and drop:  This is the feature that I enjoy the best so far.  When you are on both block and menu administration pages, you can drag and drop the positions of various elements in real time.  This use of AJAX greatly enhances the power of these pages, saving the site admins a ton of time and clicks.
• Views:  The Views module has been greatly revamped for Drupal 6.  The user interface has been drastically altered, and most people seem to like it.  However, I have found it somewhat confusing.  Admittedly, this is probably due to my familiarity with the onld system.  I hope that once I get comfortable with the new AJAXified system I will be more effective with it.  I should also note that Views seems much more powerful now, as it can produce lists of many things other than nodes (e.g. taxonomy terms, users, etc.).
• Missing modules:  There are still tons of modules that have not been ported to Drupal 6.  One of the most important to me is the Imagefield module.  I hope that it will soon be ported over.

That's all my thoughts for now.  I will update the blog once I've finished the site.  It will be pretty cool.