Sonic the Hedgehog starts of fast, and changes the platforming genre forever.
Sonic was born and instantly gave SEGA the boost they needed. Running as fast as the Mega Drive would let you, smashing robots and excellent platforming. This was the go-to game in 1991.
However it was not all good. After starting off fast and blasting through 3 stages with ease, you slow down. The game switches around like that, speed, platforming, speed and so on. It was this balance that stopped the game becoming bland. With all that though, you had a large environment to explore with secrets abound in every corner, i'm still discovering some today. It's this exploration which, while not a key game-play aspect, has players coming back.
On the topic of replay value, the series staple "Chaos Emeralds", give you another reason to collect Rings. Holding on to 50 of them at the end of a stage nabs you a chance to warp into the Special Stages via Giant Ring. These stages are tough, and while sure to make you slam whatever controller your using these days to the floor, it is worth it. Collecting all six Emeralds gets you the "Good Ending", which is lots of flowers and a frustrated Robotnik jumping angrily.
On the topic of the fat man, Dr. Robotnik (or Eggman), attacks at every opportunity, each time with a contraption to bash Sonic left, right and center. These bosses vary in difficulty, from the surprisingly easy Green Hill Boss to the frustrating Labyrinth Boss. As a bonus you get to blow him up at the end of the game! Or you can let him escape, it makes no difference.
Once again the Mega Drive sound font blows our minds as amazing and unforgettable compositions haunt us in our dreams (Spring Yard Zone especially) and we can't help but feel eerily happy at remembering them.
Overall, there is nothing wrong with this game apart from Sonic feeling a bit slow in various zones, and the occasional unfair boss fight, but don't forget the little Blue Blur is still going today.
Pros- Excellent speed
Great soundtrack
Replay value
Cons- Slow platforming