Funny I find this thread, I just played through Sonic 2 and 3 over the past few weeks. My sister and I spent way too much time on Sonic 2 as kids, but I've got to give it to Sonic 3.
Sonic 1:
+ Slightly faster Mario clone.
+ Crisp visuals and catchy music.
- Can't spin dash.
- No 2-player coop.
Sonic 2:
+ Awesome spin dash ability = much faster game.
+ Crisp visuals and catchy tunes.
+ 2-player coop and versus modes.
+ SUPER SONIC! DUH!
Sonic 3:
+ Very lively zones.
+ Mini-boss at the end of each zone 1.
+ 2-player coop and versus modes.
+ Tails can FLY!
+ Super sonic.
+ Tons of bonus maps.
- Takes a bit longer to get Super Sonic.
- Visuals don't look as sharp.
Sonic & Knuckles:
+ Interesting and lively new level designs.
+ Epic boss battles.
- No coop... IIRC.
My assessment of Sonic 1 and S&K are from memory.