On the Fly Preview – If you're making changes to a page, you no longer have to save it before seeing what it'll look like. Now Joomla 1.5 offers a quick preview button that allows you to see what your new page will look like without having to save.
Improved Menu Management – Joomla's menuing system received a significant overhaul in version 1.5. With the new version of Joomla, you can add a new menu item with just three clicks.
New Media Manager – The media manager that is a part of Joomla received significant improvements in Joomla 1.5. There's a vastly improved user interface that provides thumbnail and detail views of your content. You can also upload multiple files at once with easy and there are now some configuration choices that you didn't have before.
Simplified Extension Manager – Now there's just one installation page that works with all of them. You just have to pick-out your file, and Joomla will determine what type of extension it is and install it in the appropriate place for you.
New Authentication Methods – Joomla 1.5 now supports OpenID as well as LDAP, so you can integrate your existing user accounts with your new Joomla website. This is certainly a godsend for corporate intranet websites.