My girlfriend watched a video of me playing Final Fight and mistakenly identified one of the 'punk in baggy pants' characters as Jet from Streets of Rage 2, which she and I have played. It took a second to explain that this was a different game and it was just a common hairstyle at the time associated with street toughs.

Super Mario Brothers 2 adds more than just graphic changes to Doki Doki Panic, it adds the mechanic of becoming small (from Mario) the mechanic of holding B to run (from Mario) and it has music from SMB1 in the Title Screen (water level theme) and Sub Space sequences (ground level theme, aka The Theme Of Mario). In addition, Doki Doki Panic had to be completed with all 4 characters in each stage for the ending to be unlocked. Super Mario 2 just says "If they beat 7-2 and enter the door, begin the ending." Mario 2 then tells you how many levels each person beat and who contributed the most victories.

Other than that it's just graphic changes, but that's an awful lot of Marioing it up.