Earlier quoted context omitted.
Yes, the noticeable difference is Gimp sucks!! Hats off to the Gimp development team for pulling off so many features and above all having an open-source product out there. But the problem is PhotoShop has set the expectations way too high. And I'm afraid, a non-commercial product will never be able reach that level.
Are there any arguments why a non-commercial product (not necessarily Gimp) will never be able to reach "that level"? This looks suspiciously like "open source software is always worse than closed source!!!!!" which has been refuted more than once.
But for end-user applications where you pay for the competition and where the typical end-users are not developers, or similar, (e.g. MS Office, PaintShop, most games) then FOSS versions seem rather poor by comparison.