Sonic the Hedgehog 2 successfully builds upon everything that was great in its predecessor, but eliminates entirely only a few of its problems, while adding some interesting new ideas (like co-op) into the mix.
Pros: Incredible level design, great platforming, awesome variety in gameplay and scenery, plenty of levels, when everything clicks together (which is more often than not) it's a platforming bliss, great sense of speed, entertaining boss fights, co-op, good music, this time around there's an actual story
Cons: First few levels feel like a safe retread of the original, music is not as catchy as the original's, mix of fast action and slow platforming still has issues, slippery movement is still problematic in the slower levels/sections that require pinpoint precision, no save function/cheat codes to access levels in its original release, while it was full of devilish design decisions that would often spell death on your first time around, bullshit last level