I'm pretty sure I've beaten all the mainline Genesis Sonic games at this point... Sonic CD and Sonic 3&K more than Sonic 1 and Sonic 2, though.
Sonic 1 really is a bit of an oddity. The lack of a Spin Dash is probably the most glaring aspect, but it generally feels less refined than its sequels in pretty much every respect. Much slower-paced than its sequels overall. Also, I swear some of the things it does seem a bit out-of-place, considering later entries. Hiding an entire secret path in Labyrinth Act 1 with a floating platform that doesn't appear until you've dived down and hit a switch that does something else, without any indication? Scrap Brain 3 practically belonging to another zone entirely?
Also, barring Knuckles' campaign in Sonic 3&K (which was surprisingly difficult), it's also easily the hardest of the lot, too. Green Hill and Marble are pretty much piss easy, Spring Yard is fairly easy, Labyrinth is frustratingly difficult (if only because of that boss, egh - that was the major roadblock for me for most of my life), Star Light is back to being rather easy (provided you're decent at platforming, else you might fall into a pit), and Scrap Brain is hard, but not as hard as Labyrinth (even the Labyrinth section, considering the shortcut you can take almost immediately after starting). Also, I cannot for the life of me remember the layouts of the special stages barring the first one and the first "wide-open" one (for lack of a better word), while I could probably play a good chunk of the Blue Sphere ones in my head.
It's just kind of... weird, looking back on it. I can't really put it into words.
Thanks guys.