Can the blockchain be copied over from the Windows client, or must it be completely downloaded again?
I've honestly never tired... but I would suspect yes. I'll try myself later this evening
I've just installed the new CLAM Wallet on MacOSX Yosemite. I loaded the application and it started to download the blockchain. I closed the client and then I was able to copy across the blockchain and wallet from my Windows machine. Files were copied from Windows (%appdata%\Clam) to MacOSX (~/Library/Application Support/Clam). I restarted the client after the copy was complete and the blockchain and wallet were available and in sync.
The only thing I had to change in the new client was to stop using the black visual theme (under Options => Display) so that I could see the mouseover text for the locked/encrypted, staking, network connections, up to date icons (in the bottom left of the window). When using the black visual theme, the transparency was hard to read. I tried it against my dark wallpaper/background and against a light background but it didn't really make a difference. I do like the black theme but I can't use it atm.