Also ---- the Lr5b catalogue can't be exported and then re-imported back into Lr4 should you start a project and then decide to move that project to Lr4. I assume you can export the final images and import those into Lr4... but I haven't tried.
I have found an oddity in the Develop module where on occassion I get the zoom icon (and functionality) instead of the localized masking brush. Other than I haven't found any problems. Though I haven't really pushing it hard.