I agree with shared code base for OS X and iOS, which will help to make more cool application for OS X. But I don’t agree with intention to bring standalone apps for all service (Facebook, Inbox, etc.) to OS X environment instead of browser.
The main idea is that kind of services are for information consumers and Apple has iPads for them. You can find a lot of people who uses only iPad as their main and everyday “PC”. It’s more than enough to stay connected with friends, read and answer on emails. You can even make videos or compose music on it, I’m not talking about consuming movies, TV series and listen music.
On other hand, OS X and Macs are supposed to be used for a bit more complicated activity, such as write software, or create really complex movies, create large 3D models or work full time on designs/illustrations.