I really ate the first 5 of the Guardians of the Flame novels up as a kid (but I didn't keep up with the later 5 books published later on in the 90's). It certainly was more mature than I expected, and it hit me early on in the first book when the group got to the city and there was a certain scene that made the consquences of their situation and actions very real for the group (no spoilers). And the slave/rape stuff really made me think, and helped me process the mentality of the injustice, and developing the will to fight it, rather than turn a blind eye.
It was probably in my top 5 fantasy series back in the 80s. (Shannara, Dragonlance Chronicles, Belgariad, Guardians of the Flame, and Forgotten Realms). Tolkein didn't grab me back then, funny enough.
It absolutely was D&D with the serial numbers filed off. I seem to recall that the "Levels" of the mages where alphabetical, rather than numerical, and if you didn't have your spellbook you were HOSED. Potions of healing were very important because healers themselves were rare.
Walter Slovotsky, Ahira Bandylegs, Andy Andy, Karl Cullinane... I think I should read those books again.
Thanks for triggering this trip down memory lane!