>>715804527 (OP)A lot of the levels go on too long, and their complexity adds little to nothing to entertainment value. Most of the stages overstay their welcome, and don't really reward you for exploring anyway.
Contrast this with Sonic 2's flow style of stage design and gameplay. A run through a stage with charming obstacles and enemies woven into it. Sonic's about flow as much as he is about overcoming obstacles and Robotnik. Lose the flow, and you've lost a core part of what makes Sonic great and fun. Overly complex design is antithetical to his design and character. If you wanted complex level design that rewards exploring, then make the game about Knuckles or Tails.
Minibosses add little to the game. They're not a detraction, but they're not particularly interesting most of the time either.
The music is hit or miss. You get some bangers like Ice Cap and Flying Battery, but then most of the rest are forgettable.
Worst special stage in the series. I didn't think they could do worse than 2's special stage, but they pulled it off.
Yes, at least shields are cool.