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Ignoring the lack of updates if the game is buggy, games back then were also more focused on quality and make gamers replay the game with unlockable features based on skills, not money. I can't count the number of times I played Metal Gear Solid games over and over to unlock new features playing the hardest difficulty and with handicap features, and also to find Easter eggs. Speaking of Easter eggs, you'd lose a number of hours exploring every nook and cranny finding them!
~~Actually, OP very explicitly said to ignore bugs and was only talking about gameplay. Which is why they talk about extreme replayability being the requirement on old games.~~
I just realized you were talking about who i responded to, not OP. But still, they weren't only talking about bugginess.
The basic mechanics of a game (eg. Mario) better be fun, and those first couple of levels better be fun, because that's what you'll be doing a lot. It's similar to how the swinging in Spider-Man better be fun because you'll be doing it a lot. But the it also has more complex fighting, side content, and a story. You can mess up a lot more while there's still enough to keep it entertaining.
But people don't remember the majority of games that were not very good. World Games was just a game that came to mind on the NES as being not very fun, but more importantly forgotten.