Hello, I am so happy GIMP finally has a single window mode. The old user interface used to drive me nuts. It was just cumbersome on any OS I used it on. I think for new users trying out GIMP for the first time will see the default UI, and run screaming for the hills.
I feel the installation should have the Single-Window Mode selected by default, because people might not know the option is there. A lot of people are coming from other tools, none of which I have seen a UI as bad as GIMPs default one. For the people who enjoy the classic setup of it, they will know about the option, and know how to disable it right from the start. Which as I said before new users may not know it is even an option to look for. It is REALLY a turn off. Please take this into consideration for retaining new users, and letting them learn just how powerful this software is. It is sad some install it, take one look, and never touch it again.
With that said I love GIMP. I have used it for many years now, and I cannot tell you how happy I am with 2.8.X. There is nothing I found missing from it that other similar programs offer, and if there is the wonderful people making plug-ins (likely) have made it possible. Thank you plug-in community!