If you have an editor that people like as much as TextMate then there will be no major reason really that OS X will be better than any other *NIX, but the main reason I love my Mac is like most other Mac Developers. The interface is nice, it's a very stable platform to work on, ie I spend more time working rather than getting it to work, and also TextMate. For all development that I have done there has been no better editor to use that it, and when you feel comfortable with your tools then you will be at your most productive. I think that this is the reason why people love to develop on the Mac.
