One of most annoying limitation SEO wise in the Joomla versions prior to Joomla 1.6 was the inability to set directly (without use of some SEO component or a core hack) meta data information for categories. This was important especially on category blog or category list type of pages, where you have left without proper way to control the meta data added to these pages.
In Joomla 1.5, you were unable to add specific meta data to the section and category pages. That's been fixed in Joomla 1.6. You can now add not only meta description and keywords, but also an 'Alternate page title' as well as the author and robots tags. It's one of the most welcome new features SEO wise in the new Joomla implementation!