AFnord: Oh, after re-reading the original thread (after reading Kalirion's bold text), I realize that I might have misinterpreted it a little.
I've not bought a single game on GOG which I previously owned. I did own Beneath a steel sky & Ultima 4 though, and had beaten them before joining GOG.
Yeah, I think I've misread the first post as well.
The following GOGs I've started (and put at least an hour in), but then stopped playing:
Rayman Forever (too hard, I may complete some more levels but definitely not Mosquito's Nest w/o cheating)
Baldur's Gate 2 (stopped playing before the finale for some reason, will likely restart at some point)
Descent (got too hard and annoying, I still occasionally play it a bit to see if I can progress)
Beyond Divinity (got too annoying)
HoMM 3 (was taking too much time per scenario, I'll probably go back)
Ishar 1 (I'm not much for making my own maps)
Blake Stone: Aliens of Gold (will probably come back at some point)
Rise of the Triad (see above)
Expendable (couldn't stand the controls)
Aquanox (see above)
Arx Fatalis (too hard to id the loot)
Stonekeep (just stopped being fun)