It does not appear that the new version has anything too exciting, not even Push notifications or Facebook chat. The auto refresh setting has been renamed to "Cache expiration" and the notifications listing has a noticeably smaller type, although you can now touch the whole notification to go to the related post (rather than having to touch a small part of the notification update).
I personally do not own a Windows phone so the above observations are just what I've heard from friends and colleagues. I have contacted Microsoft for an official changelog and I'll update this post if a spokesperson decides to reply with something tangible.
"Facebook for Windows Phone 1.5 available now," a Microsoft spokesperson said in a statement. "The 1.5 release includes a lot of fixes to known issues and significant performance improvements. There is no net new features in 1.5; we are working on those still and will share our plans for new features once we have enough feedback on this release. Thanks for your patience!"