AFnord: There are of course many games that have stood the test of time, that are fun 20 years after their release (Ultima 7 is a good example), but I would say that most are not, in particular not the very early entries in a genre.
Funny you'd say Ultima 7, because I've felt that if I replayed it today, it wouldn't feel even as close as wonderful as "back then", with all its bugs and user-interface issues. I don't know if GOG version has the same issues.
One game that I always want to take as an example of a game that has stood the test of time IMHO is another Bullfrog game, Magic Carpet (and now that I have played the sequel again, it feels good too even though I found it inferior to MC back then for some reason). And this was a bit of surprise to me because one of its major selling points back then was the incredible graphics system, which were about as revolutionary as e.g. Doom's or Ultima Underworld's, or Comanche's voxel, graphics engines.
Since the graphics are nothing to shout about anymore and I find it so enjoyable even today, apparently it wasn't all about graphics then. I'd love to see some kind of remake or similar game, ie. free form action game with light strategic elements.
Psyringe: Imho, the weaknesses of typical Bullfrog designs are also the reason why they didn't age particularly well. For their success, nearly all Bullfrog games relied on two common factors: novel ideas, and great graphics. Gameplay, and especially long-term motivation, often took a backseat.
I guess I still disagree for Magic Carpet. :) The gameplay is mostly quite repetitive, albeit the further you play, the more special levels you get which are not only about "make a castle, kill enemies, build more castle..." etc., but they were more like puzzles. And even if fighting the enemies was repetitive, there was always the strategic things to consider (where to build the castle, should it be replaced to another location mid-game etc.), the urge to get the better spells activated etc.
One gameplay element that I definitely praise Magic Carpet for was that usually the most challenging parts of the game were in the beginning parts of each level, meaning that you didn't have to replay long boring parts just to reach the point where you died the last time.
Somewhat moot point though, after I found out that there was a way to save the game during levels. :) Maybe I didn't read the manual too well the first time, or was it some undocumented feature... Anyway, I think I played the game through the first time ironman mode, ie. not saving during levels, just because I didn't know better.