There seem to be three distinct classes: sequels, revisions, and modifications. Sequels are new games, even if they aren't always any good. Revisions are minor, and basically have no play difference from the original. Modifications are major changes that make a real difference to the gameplay.(posted 9780 days ago)Sequels, like Street Fighter 2: Champion Edition or Super Zaxxon, have different gameplay and graphics that make them a new game. Some of the pirate or prototype boards, like Piranha or Trog, aren't really the same game and need sequel status.
I get the most annoyed with the minor revisions that don't affect gameplay counting as seperate games. Examples of this I can confirm are Mat Mania/Exciting Hour, 10 Yard Fight, and Discs Of Tron.
The most blatent revisions are like Mat Mania/Exciting Hour where you can play back a recording made with Mat Mania on Exciting Hour and vice versa. I think in cases where recordings are compatible, we should pick an "official" version and treat any recordings that play back on that version as acceptable. I'm not sure how common it is because I haven't tried it much.
Modifications are probably going to be the most fought over, because some of what the game designers considered fixing us players considered breaking. The best example of this is Asteroids, where the saucer hunting was made much harder. I could never really do the trick well, but to me Asteroids just isn't the same without it.
I wouldn't count the versions of Donkey Kong with different level orders as an important enough difference to overall scoring, but I'd expect the ladder bug would cause a big difference in scores.
So much for easy :(