To Mega 2: Super Mario Bros. DX includes an edited version of the Lost Levels(Super Mario Bros. 2 in Japan) and some extra stuff like a photo album, a race againt boo, and a fortune game. As for the game SMB itself, you can save(Yay!) in up to three different slots, and instead of shrinking the graphics tiles to fit on the small screen, they've just made it so it scrolls up and down(via Up and Down buttons or when you move). Hope that answers your question.
To Doomba: Yeah, that'd be great. I was just thinking that. I really like the SMASDX idea. If only the GBC had more colors, they could fix up the graphics, but if they do anything involving any other Mario games, they should just shrink the tiles, not knowing what's on the rest of the screen can be annoying at times.
-YOSHI :P