Even with just gnustep-core you have enough power to write a portable (to the mac) application of the caliber of a full office suite. It was done on OpenStep, it oculd be done again. Of course this is not a "direct porting". But if porting is desired, it can be considered in the application design phase already. This is done for other libraries like GTK, QT or Wx.
Very true!
Still, the problem remains that existing apps are not necessarily easily portable ("just recompiled"), that the target environment is not very receptive for mass-market commercial apps, and that even the best planning would mean that one platform would be underserved, or that development time might prolong. If I develop a commercial blog writing tool, what kind of a market will I have on GNUstep? Will I be able to "just use" various eyecandy API available on OS X? :-)
* that does not mean it cannot be used to produce quality apps -- because it can, and hopefully we'll all prove it!
* that does not mean it is not powerful -- because it is! On the developer side, and by its design, I strongly feel it is the most powerful FLOSS environment. (I hope to try out Etoile one day or the other and see how far along is the user experience in a GNUstep-based desktop environment.)