A common question in upgraders’ minds seems to be whether the old plugins will work with version 1.5
A lot of them will, some won’t. Here’s a brief overview of what might break. ‘Plugins’ that required you to edit source files other than index.php may be broken when you upgrade, overwriting the files. Some plugins might be affected by the restructuring of the index.php page, if you decide to use the new all-split-up index.php. The change involved in using the $wpdb class to access tables should not affect any plugins that use the old method (of using one variable for each table) since backward compatibility was maintained when the change was made. Plugins that use a menu page of their own might break. So would plugins that used or referenced files or options that are no longer a part of the core, like the geo-* functions, and the links-rating-images.
So good luck with getting the plugins working.
I urge plugin authors to upgrade their plugins, and check if everything still works, and user to urge plugin authors to do the same.
I know I have a couple of plugins of mine to review.