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• Cost of consultants to keep your PCs working. Macs are just a lot less trouble.
• Cost of your time dealing with malware. Yes, Macs can theoretically get compromised, but it's just not happening in reality. And it's unlikely to happen on the scale seen on Windows due to the Mac's better security design in general.
• Time cost of dealing with clunkiness of Windows/Microsoft software. This is a generalization, but time after time I find complex multi-step interfaces for functions like installing software or just saving a file. When instructions are provided for doing the same task on Windows and Mac, the Mac instructions are usually shorter (at least if the writer knows the Mac).
The main obstacle to switching I run into is the cost of purchasing Office for Mac. But for the vast majority of users, Apple's iWork suite, which now includes a spreadsheet app, is all they need. $79 a seat, even for business users (and a 5-user family pack for $99).
Macs aren't perfect, just very good.
P.S. to Miette: Macs have shipped with a 4-button/scroll wheel mouse (the Mighty Mouse) for about two years (which, to help newbies, physically seems to be a one-button mouse until you set the other buttons).