Childhood nostalgia absolutely plays a role, but in my experience game developers tended to trade quality for graphics as the years advanced. In the 1980s games like Ultima 4 couldn't use graphics as a crutch and thus had to make the games really engrossing. My other grips is the ever-decreasing difficulty of the games. Back in the 1980s most games were difficult (if not very difficult/nearly impossible) to finish. You could play a game back then for months or years and never finish it. There was no internet to search for the answers to problems or riddles you couldn't solve. Games were not designed to be easily completed by virtually anyone who played. I can't remember how many hours I spent playing Zork before I was able to solve all the puzzles and beat that game (let alone Zork II and Zork III).
I'd say that in the 80s most game concepts were invented, but many suffered from the technical limitations of the time. In the 90s, most of these limitations were gradually lifted. The high point is probably different from game category to game category.
For instance Pac-Man, Tetris, or Galaga are still good fun today. Ditto Super Mario Bros. Flight Simulator II? Sorcery? not so much. There are plenty of 3D games from the early 90s perfectly fun to play, and some IMO like "Thief" haven't been bested in gameplay.
