I went out and bought a Mac Book Pro which had Mavericks running on it and upgraded to Yosemite (Mac OS X 10.9). I felt right away that support for Open Source was reduced and that the App Store was made more visible, most of which you pay for.
I tried to install a number of open source programs that have Mac versions. First, I had to turn off an option that disallows non App Store programs! There are a couple of well known programs that Apple will not let me install into Applications and run from the Launch Pad. Those programs can only be run from within the disk image files they shipped with. They are Celestia and Stellarium, two planetarium programs that ship with and run on all Linux systems.
I was able to install two versions of Emacs, both GNUemacs and Aquaemacs which is based on Gnu emacs 24 as well. The first does not save its settings and so I have to manually set a larger font to use it. A way around this has been to start terminal with a large font, which Aqua remembers, and run emacs 24 from within the shell. I had to find the path to the binary in Emacs.app and run it from a shell script. That works because emacs is designed to accept arguments from the shell.
I also installed anaconda, a big scientific library bundle with python and have been able to use ipython notebook from it. That was a major goal in getting a Mac.
Not only does Apple try to dissuade from installing third party software not in the App Store but you are required to get an Internet login at Apple called an Apple-ID to upgrade your system at all. There is a gotcha that is not well enough documented that it is possible for core apps to be installed with a different Apple-ID then the one you set and you will be denied the ability to upgrade and the resulting message doesn’t really tell you the workaround, which is to go to the store menu and override the login for each app, individually.