Well, I'm in the process of doing a playthrough of all the classic Sonic games on Genesis (at least, re: 1, 2, 3, and CD), so I feel like this thread is a good place to relay my thoughts once I was done playing the game.
Anyway, I've played through the first Sonic game, being my first 2D Sonic sidescroller. Thoughts...
It's not the game's fault that it feels like a 1990's Genesis game, but speaking from the perspective of someone who's never experienced the games as a kid, it's very 1990's Genesis game. Your lives serve as your continue counter: you run out of lives, you start the game from the very beginning, all the way back in Green Hill Zone. Fortunately, in this day and age, Sega has included the option of save states as an option for players who simply don't have the patience to play through earlier levels just to get to the part they lost all of their lives in. I wouldn't exactly say this game is very difficult, as it's fairly easy to amass lives in the first levels, but running through the levels the first time can be pretty miserable, especially if you expect Sonic 1 to be a game where you can just blow through the levels fast and then fall into a pit or get hit by an enemy. Its main selling point, Sonic's speed, I feel like is underutilized at points and many of the levels you're better off taking it slow since you're unfamiliar with the layouts. There was one irritating part in Scrap Brain Zone where you speed through very fast, so fast, and you're enjoying yourself, and then you get slammed by a random bomb enemy you can't defeat which is impossible for you to know ahead of time, and I find that enemy placement to be very poor. Pits are also telegraphed very poorly, you can sorta get an idea of what parts of the levels are bottomless pits or not, but it's not very clear, hence how I don't like dropping down levels if it's potentially a lose elevation or a bottomless pit. The experience with the levels improve, however, when you play through it again, and it truly gets satisfying knowing the layout and then taking advantage of your speed. The biggest advantage of the levels however, is the high amount of multiple paths you can take, making the game very replayable as you can take a huge variety of different paths to complete a level.
How does Sonic defeat enemies? He has to be in ball form. Unlike Mario players, Sonic can jump right into enemies to defeat them, but the catch is that Sonic can't fall down and KO enemies. In order to have a safe landing, you have to jump as Sonic and fall down while he is in a ball. Sonic can also roll into some enemies to defeat them and it helps him gain speed down slopes. Sonic can collect rings in the level, which serve as his hit meter. Sonic loses rings if he gets hit, and if he has zero rings and gets hurt, he loses a life. Some places have him die instantly such as getting crushed or falling down a pit or drowning. If Sonic activates a checkpoint, if he dies, he can respawn there. Sonic can find some few power-ups in the level, such as invincibility, a shield that protects him from losing rings if he gets hit, and something that increases his speed. If Sonic collects 50 rings and finishes the level, he can enter a challenge level. The challenge levels, you navigate through a maze in ball form to try to get the Chaos Emerald and avoid the markers labeled as "goal", and I found these levels very disorienting to play in and...ehhh. However, if you want the good ending, you need to do this challenge six times to get all six Chaos Emeralds.
Oh and there's a 10 minute time limit for every level sorta like Mario but it's almost nonexistent and it's pointless. 10 minutes is more than enough time to complete a level.
Speaking of the zones, you have several zones with three levels (called acts). Once you played through them, you can input a level select option to replay any specific level you like. At the third act of every zone, you fight Eggman (Robotnik) with his mech. You jump into his mech multiple times to kill him while every zone has him with a unique Eggman gimmick, with Labyrinth Zone in particular having you have to beat the water and the obstacles rather than actually fighting him.
Green Hill Zone is a good starter zone and a good place to stock up on lives, but don't expect to run fast through its levels, you'll find spikes and pits and some bots. It's a good place to introduce to some fundamentals of a Sonic game, including taking multiple paths and exploring the level as you please, at least under a 10 minute time limit.
Marble Zone is a zone where you need to take it slow, ride on platformers through the lava, and avoid lava-themed obstacles in a dungeon-like environment, almost reminiscent of traveling through a Castle or Fortress in a Mario game. The caterpillar robots are annoying as you need to precisely land on their tiny head to defeat them and if you touch any of their other segments, you get hit. I liked this zone quite a bit actually, but perhaps because it reminded me of Mario games.
Spring Yard Zone has a lot of platforming by navigating a bunch of moving platforms, and has a lot of slopes and you guessed it, springs. It's a zone where you can travel through pretty fast like Green Hill, but it has a lot more platforming segments.
Labyrinth Zone is the worst zone in the game. The water levels feel really bad to play in. It slows down Sonic's speed to molasses, including his jumps, making platforming and navigating a slow and painful slog and working against Sonic's strengths, and Sonic is in a time limit when he's underwater, as he needs spawning air bubbles to restore the limit. The air bubbles spawn at a hugely slow and inconsistent pace: there was one point in the game where a bubble spawned right when I drowned. You'll spend a lot of time just waiting for bubbles to spawn and I hate it, they should spawn at a consistent pace. Enemies underwater slow down as well, and the gravity of the water makes it harder to deal with the Orbinaughts, enemies that toss spike balls at you. It's not a fun zone, period. At least the music is nice. That intense drowning tune though. Yeah no wondered it scared kids.
Star Light Zone is a return to platforming and speed, and I find it similar to Spring Yard Zone in terms of a stronger emphasis to platforming. It has an annoying fan mechanic that pushes you around, and the introduction of the annoying bomb enemies. I didn't find it too memorable. Hell, when I reread it to remember its gimmicks, I didn't even notice it had a lack of checkpoints to increase its difficulty.
Scrap Brain Zone, the final zone in the game, is the Eggman Zone and it has a lot of traps and gimmicks, such as the transporting elevators, alongside its intense soundtrack. Pretty fun and challenging to play through I must say, even if the flame pillars are sorta annoying and not too clearly telegraphed, as the vents blend in with the environment.
So overall my thoughts after destroying the fat man? It's decent, especially for its time. The graphics are very pretty and give the environment an organic flair, which is points I'd give it over Mario's more blocky environments. Its soundtrack is mostly inoffensive but still a bit memorable. I've had a taste of Sonic 2 already, and people aren't wrong when they say the next games are an improvement over this one in every way. I still suggest players new to Sonic Genesis games to start with this one and work their way up, they'll learn to appreciate the improvements and enhancements to the Sonic foundation the next ones introduced. Can't wait to play through Sonic 2 next!
Pros
+Beautiful graphics, organic, beautiful environments
+++Tight gameplay with easy to learn controls that carry through the entire game
+Game set the foundation for future titles, which owe their success to this game.
+Has a great sense of speed
+Fun, creative zones (except Labyrinth Zone)
+Decent soundtrack
+++Well-designed levels with many ways to complete the zone thanks to the amount of paths you can take.
Cons
-Outclassed by future Sonic games, especially 2.
-Plays like a 90's Genesis game, with a lot of archaic elements and rough level quirks associated with it
-Challenge levels are disorienting and mediocre
-Very short, you can probably beat this in a day, even if you do die and start in the very beginning.
-The entirety of the Labyrinth Zone
-Doesn't take advantage of its speed very much.