Ranked in these QuestionsQuestion Ranking
Pro Based on Unix
Being based on Unix means you can install a lot more stuff with a lot fewer headaches then if you were on Windows.
Pro Powerful terminal
It's very similar to a Linux terminal.
Pro Polished UI
Pro Easy access to lots of great dev tools
There's a large selection of great development tools available for OSX. The operating system itself comes bundled with a powerful terminal emulator, called Terminal. Additionally, Apple provides tools, like Xcode, an IDE that contains a comprehensive collection of tools for developing OSX and iOS software, for free.
Pro Lots of open-source software available
Because it's Unix under the fancy GUI, most open source ports easily to it.
Pro Best support for Objective-C
Pro Great Git GUI tools
Tower, Kaleidoscope, SourceTree
Pro Has too many special tools for devolopers
Pro More commercial software and gaming support compared to other Unix systems
Adobe CC, MS Office, Steam games.
Pro Ideal setup, out of the box
Next to no custom configuration is necessary.
Pro Great Modifier key layout
OSX is tied Apple hardware and Apple hardware tends to be expensive.
Con No native package management
Con Most software is closed source
For people who like to use open source tools for their development work, this may be a problem. There's plenty of advantages to open source software, one of which is the ability to tinker with and customize the tools themselves that you are using. Although there's plenty of FOSS tools available for Mac, especially through Homebrew, the number of packages available is much lower than the number of packages available for any Linux distribution.
Con Very few options for running hosted, on the cloud
Con Silly modifier keys layout
The Command key is strange, Alt is where Super should be.