A brand new Mac mini today may be a great machine, but if I had bought an iMac two years ago I certainly wouldn't have much inclination to buy a Mac mini today. Setting aside the question of the included display v. buying a new one, the Mac mini of today is not significantly faster or better than a two year old iMac. After eighteen to twenty-four months of Mac mini ownership I probably would be looking to get another one, whereas I can see keeping my current iMac for three to four years. I'm not saying that the Mac mini is a bad computer because I certainly don't think it is, but if anything it has to be equated with an iBook, not an iMac.
There's the other consideration, too, of the software I want to be able to run. As you may have picked up from another thread I am an Aperture user, and Aperture simply won't run on a Mac mini. In fact, I bought the iMac based on being the best computer that I could afford and that would run that software.