Thankfully the list of XPage fixes we've added to 851 FP1 isn't as long as it was in 85 and the following are well worth mentioning all the same.
# PHAN7X8T85 | Problem with actions in nested custom controls:
Top of the list is this one and you're likely to hit this if your application uses nested custom controls. For example, you might have an edit document simple action in a custom control and this custom control is embedded in another, it might be in an Editable Area, and at runtime you find that have to click the action twice for it to perform the action. On some applications the action won't perform at all. So if you see this happening to your XPages application on a 851 runtime, apply FP1 to resolve this issue.
# MKEE7V9F82 | Browser Time Zone always shows GMT:
You'll only hit this bug if your application is set to use the browser's time zone (this is a setting you can enable in the Application Properties on the XPages tab and select 'Browser' for Time Zone) rather than the servers (default). This has been fixed in FP1.
# PHAN7WSQRP | Unable to call the Quickr API from XPages:
This is another issue resolved by FP1 and it should unblock you if your incorporating Quickr into your XPages application.
Hope this helps.
Thanks for the info. PHAN7X8T85 has been causing me some trouble, will see if this helps.