There were no improvements under the hood (though even the lowly Nokia Lumia 610 was fairly smooth on WP7.5, and still is), no OTA updates, you're stuck with Internet Explorer version 9 instead of the much zippier IE10 and apps created for WP8 won't work -- giving developers very little reason to create new ones for the lame duck OS, or even update existing ones.
Also about synergy power betwen them perhaps I would add the fact that together they dont overcome their competidors in nothing exept for build quality Nokia phones have and the new concept of UI WP7 has and some services like office wich has not atracted many users yet.