hehe
Its linux and it will take some getting used to.
Its whatever you want !!!
You don't even need a dm at all you can do it from a CLI
I think KDM has more themes ...
However non of them touch OS-X for changing users, very cool 3d rotation but just eye candy!!1

I think what you need to do is a HDD instal then you can start playing....
decide what you want etc.
themes.kde.org is a good place to look.