• Keyboard: standard PC layout, remappable ctrl/capslock with the capslock indicator indicating capslock, not n%2 physical key operations, pageup/pagedown, home/end, arrow keys, delete and backspace.
• Trackpoint. I hate trackpads.
• Three-button mouse.
• Choice of window manager, for:
• Focus-follows mouse.
• Mappable window manager hotkeys. I've got a set of features I've programmed into my window manager over the past 17 years (ironically: based on Aqua's antecedant, WindowMaker). They fit my workflow. Among them: alt-shift-t pops up a terminal window, alt-shift maximizes a window vertically (what the keybindings are matters far less than that I can map these functions. Resizing windows on OS X is a goddamned fucking mouse-up-your-ass-but-only-the-lower-right-corner pain. Every. Fucking. Time.
• Pinnable window menus. For when I need to track down a specific window (xlsclients | wc -l => 118 -- no, Expose doesn't cut it).
• An integrated package system with an unparalleled set of packages: grep -h '^Package:' _Packages | sort -u | wc -l => 50493. Yes, 50,493 packages. OS X has fink, and darwinports, and a mash of other tools, but fails to deliver a single integrated package management system, and the offerings I've found are sparse and tend to be poorly supported.
• Customizeability, in general. Not tweaking for its own sake, but simply getting things to work how I want them to.
• /proc and /sys. Believe it or not, having filesystem-based access to system internals is seriously fucking amazing.
• No "undocumented" features. Which isn't to say that there are system features which lack documentation, but there are not features for which documentation is being intentionally withheld*, as is the case for Macs.
• My hardware won't age out while it's still remotely usable. I've got 22 year old systems I can still boot and update. They are barely useful (network infrastructure), but they work.
Mark Pilgrim had a great "switching to Ubuntu" piece that got pulled when he deleted his website(s). Cory Doctorow points to it and mentions some highlights here: http://boingboing.net/2006/06/29/mark-pilgrims-list-o.html
Where I've seen Mac used extensively, a few elements seem to dictate its choice:
• For user not highly versed in Linux configuration, Macs do tend to be an easier to get-up-and-running system. Though honestly for desktop rollouts of the past 5-10 years, it's a very close race.
• There's often a use of Mac-specific applications. OS lock-in through application space is OS lock-in. It worked for Microsoft for a quarter century.
• It's often good enough. Not perfect, but Apple does very well at delivering a "sane-by-default" system. Mind, Ubuntu and Debian are getting damned good at this as well, as is Google via Android.
But to say that Mac is a foregone conclusion is hardly the case (as many here note).