I just upgraded to 1.5, and I've been slowly converting the plugins and themes over. Definitely worth the effort. One thing I noticed. In my old index.php, I had modified wp_list_cats to:
<?php wp_list_cats('sort_column=name&all=all&optionall=1&optioncount=1'); ?>
However, I'm not getting the 'all' option in the list now for some reason. I tried changing around order of parameters, and adding and removing things in different combinations. It doesn't seem to matter much. All the other parameters I add and remove seem to work just fine. 'optionall' doesn't.