So what will the new Joomla 1.6 make it so much better than trusty old Joomla 1.5? Let's go over some of the new features Joomla 1.6 is offering:

User access levels
In Joomla 1.6 it will be much easier with the whole user access levels. in the old Joomla 1.5 version, you simply had a limited amount of options to set the access level for a user. For example, you couldn't set the User 1 to have only access category A, and not category B. In  Joomla 1.6 all this will be possible, you can assaign access levels for each created user, you can create your own user group and then assaign a number of users into that group.

Less code
Joomla 1.5 has more than 100 000 lines of code compared to Joomla 1.6! So Joomla 1.6 will use less code to do its job, so in short: Joomla 1.6's code will be more efficient. I will not go over all the technical aspects of the way Joomla 1.6 write it's code, but the major difference is that it will need less code than 1.5 to perform same things.

Cleaner and simpler back-end
One big problem I felt with Joomla 1.5 was that the back-end part was a bit too complex for a newbie. Often when I have introduced back-end to clients, they have been overwhelmed by all the menus, all different areas, parameters etc. In Joomla 1.6 this will be much more clean and not as many different areas that may confuse the average Joe customer.