I've recently spent a little time porting some FSX missions over to P3D, and there are a couple undocumented (as far as I've seen) changes. Many are minor and not unmanageable, but I'll note them nonetheless. I've noticed that P3D is a lot less "flexible" in terms of what it allows and disallows, and some things that could be overlooked in FSX can no longer be (overlooked) in P3D.
-P3D no longer accepts missions without mission briefings.
-Property comparisons made in FSX (either in property triggers or as a component of proximity triggers) are automatically wrapped in "<And></And>". FSX ignored the superfluous wrapper, but P3D won't run the mission if they're there. SimDirector will point this out to you, though.
-SimDirector allows you to optionally load (legacy) .xml files, but every time I try this, P3D crashes. Legacy missions load fine using .flt files, so I'm a little confused why the option is there for .xml.
And a minor quibble - SimDirector makes a few fields uneditable in various objects, e.g. MDLGUID for scenery objects. These were previously editable in FSX, and it's useful to have these accessible (obviously they're still directly editable in the .xml; just a convenience here).
