Great applications/uses that require a Mac:
1. Todo lists (Omnifocus or Things): I manage far too many things, and need to rapidly move through my data in a way org-mode doesn't allow. A GUI, a beautiful one, is key here.
2. Interfacing with my iPhone.
3. 1Password, Alfred, DaisyDisk, Delibar
4. Honestly…I'm struggling to come up with a lot more. I really like my calendar application (BusyCal or iCal, depending on the year). Mail.app is nice, but I just haven't taken the plunge to command-line applications yet. I wonder which I'd prefer if I put in the time required.
What is holding others back?