I think what defines how well a game holds up is if the original concept is still fun today. Old Atari tank games have not held up well. Games like Pac-Man or Galaga tend to hold up well. Galaxian feels dated because of it's slow style. Galaga holds up, I think, because the fast and solid gameplay, quick difficulty ascension, and cool secret power-up help it maintain an air of modernness.
Super Mario Bros is pretty well timeless because they nailed the gameplay and control way back on the NES.
Any game that demanded trial-and-error or blind luck over skill and thoughtful playing tends to age poorly. Modern gamers don't have the patience for the kind of crap we often put up with in the 80's.
