^ THIS ^
Sonic 2 exemplifies exactly how to make a great sequel. Take what was amazing about the original and multiply it several times over, adding new depth to the experience. For example, the collection of the Chaos Emeralds in the original earns you an extension to the end sequence of about two seconds, neither of which are all that interesting. In Sonic 2 you get Super Sonic. Remember the first time that happened? Sat there as a child, having finally mastered the art of collecting Chaos Emeralds, you get told there's something called Super Sonic, you leap into the air with 50 rings, AND YOU POO YOUR PANTS.
Ironic that we could make such perfect games years ago but can't do it now. Everyone's too concerned about graphics and ripping people off with DLC to bother making great games these days. Sonic 4 is a prime example. Only four levels? No way are you getting any of my money for something that piss poor. Drop me a line when the other half a dozen levels show up and maybe we can talk, Sega.