To be clear the only programs that just install on OS X are programs already compiled for the Max, right?
And that right there limits you. But it is getting better..they even have some steam games on it.
There limits on a Windows machine too. Does Windows have iPhoto or Aperture? No. As an amateur photographer, iPhoto is the best application I have used.
There are thousands of applications for the Mac. If you would like to see several hundred of them, go to www.apple.com/downloads/macosx and click on some of the categories listed on the left side of the page. Each category has many pages of applications.