I really liked Gothic - It's definitely more on the Planescape: Torment end of the spectrum than the Icewind Dale end. (BTW, my assessment of those two games was precisely the same as yours.)
The story is good, and the NPCs are interesting (theay also move around, and look up when you approach, which is a nice touch). I liked the non-linear aspect of the quests in the first 2/3 of the game as opposed to the more linear final third, but even thesequests are a bit less linear than, say, Diablo 2, and they can be completed either through combat or sneaking about, depending on your character type. I liked that lying in certain situations or killing people had consequences, and that pretty much every are of the game was open to you right from the start, whether you are ready for them or not. I also liked the option of joining whichever camp fit with the personality I decided to play with my character, and having that choice have actual repurcussions on the skill tree that opened up, spells available, etc.
I did not like the interface (especially the inventory management - you can't type in the amount of currency you want to spend. You are restricted to arrow keys that advance quantities 1 at a time. Armor costs over 1000 ore...), but got used to it, and am now quite comfortable with it. I did not like that there are only about 5 female characters in the game, and that you can't speak to ANY of them at least in in the English version. Women, it seems, are not very important in the world of Gothic.
Gothic has quite a bit of gameplay, despite it only being a single player game. The game also has some replay value, as there are different strategies for character building that will affect how you tackle the game (though I'm told the end is the same no matter what). The graphics are very good, and the voice acting is pretty good as well. The music is adequate.
While not as good as Planescape, IMHO, Gothic is quite good, and worth the purchase price.