Single player in Smash = normal arcade fighter experience
Single player in Melee = unique concept of trying to revisit some worlds of the video game past with Melee's fighters. Wasn't the most polished single player in the world and had room for improvement.
Single Player Brawl = we know there will be cut scenes, unique stages, and that Sakurai has pushed and wanted to make the single player aspect of Smash a much better experience than before.
That's where my assumptions come from but if you would
look back on the forums you'll notice I said that it being a good single player experience would be icing on the cake. I didn't say it would be astonishing I just said it made me hyped for the game.
And then people started saying how the multiplayer would suffer . . .
Sooooo yeah =) I didn't quite stand up and say "instant greatness single player mode" or anything. I just said logically it should be a good experience based on Melee's single player game and comparing it to what they are doing with Brawl (or say they are doing).