The Platinum comes with Palm OS 3.5.2H, which adds some new features like Ir synching and a command menu.  Other features include the ability to beam entire categories and duplication of address book items.  One feature that I like is that you can now mask private items instead of hiding them.  Editing is also easier.  Double-tap to select an entire word, and triple-tap to select the entire line.

If you are upgrading from the Visor, there can be some "gotchas."  If you use BackupBuddy, remove any OS patches from your Visor before you do that last HotSync before switching to the Platinum.  There are also some applications that do not work under OS 3.5 like Launcher III.  If you have any problems after upgrading, check the app developer's website first and see if there is a more recent update.  Also, some modules like the Eyemodule and MiniJam must be updated to work properly with the newer 3.5 OS.  If you're having a problem with your module or application, contact the manufacturer.