For some people using a Mac is a religious decision, for others it's a practical choice. Redmonk analyst Michael Cote provides an excellent list of Mac-related knowledge, including applications and things you need to use/understand in order to enjoy the Mac.
I'm always shocked when people tell me they don't like the Mac, primarily because they don't understand that the MacOS is much more humanized and has a deeper level of mind-meld than Windows does.
Several times a year, a long-time Windows user I know switches over to a Mac. What with the new MBP's ("MacBookPro") out, there're several people "making the leap," as they always put it. These people are usually "business users," people using their machine for work: information users, programmers, managers, etc.
Thus, the things I care about on Macs tend to be slightly less consumer focused and more focused on things like email, calendaring, and massaging all the information that flows through the your daily work-life.
It's pretty amazing to realize how many third-party applications (meaning not from Apple) I use. In fact Mail.app is the only thing I use fairly consistently. I assume this is both a testament to the ecosystem and the fact that apps on the Mac seem to "just work."