Ok I'm the 100th person to say this today, but of course Super Metroid isn't hard when you know the game. As you learn more about it and acquire more items, it become much easier. That's where the replay value of 100%ing or speedrunning the game becomes apparent. Since your health and several of your weapons capacities in the series depends on how exploratory you are, a knowledgeable player can adjust the difficulty to suit the challenge they crave. Super even has the distinction of being able to turn off every upgrade, meaning the damage resisting properties of the second and third suits can be disabled, too, without having to do "hell runs" through Norfair or put up with the water physics.
This game and Zero Mission
expand greatly upon the player controlled challenge by letting you skip items. I played through Super once without grabbing the grapple beam. Getting into the wrecked ship and through Maridia without it is a test of your speedbooster/shinesparking and wall jumping skills. It's not too hard once you get the reflexes down, but adds to the replay value. I think someone has gotten to Ridley's lair without the space jump. (Still trying to figure that one out.) Skipping items and low percent runs are why ZM has the 15% endings.
The original, Super, Zero Mission, and the original NA version of Prime (which is easily sequence broke) have so many ways to play them, both to make it easier, or to go after stronger enemies sooner. The others unfortunately railroad you through the item progression, making difficulty settings and skipping energy tanks and missile expansions the only ways to add challenge to them.
Fact that Nintendo only made one 2D Metroid as open as open as Super in the past 21 years might be one reason why it's also a popular game for ROM hackers[footnote]Disclaimer: Hairless Mammoth does not endorse piracy (both software and naval based), "sticking it to the man" (in the form of piracy), or aggravated hair loss by playing super hard games made by sadists. He always strokes his SNES cart of Super Metroid for good luck before playing Redesign and quits for a few hours if it gets too frustrating. If you wish to try out any unofficial game mentioned by this guy in plaid pajamas, carts can still be found on ebay and are worth their prices.[/footnote] (also most hacks up the difficulty because they expect players to be series veterans). I'm playing through Super Metroid: Redesign again, and the wall jumping and pixel perfect running jumps are something even save states can't really help you with (though saving on a whim is a godsend against some reallly powerful enemies). Even if you, for whatever reason, don't wish to play ROM hacks, I'd recommend looking at playthroughs/LPs online of Redesign and Super Metroid Eris (especially Eris for it's gorgeous original environment graphics). Super Zero Mission is another one to look out for.
Mother Brain may just be an interactive cutscne, but Super Metroid does have a final boss fight -- Ridley. That fight is hard enough that I died even on my last 100% playthrough a few months back.
Indeed. Ridley is a damage sponge is most of his incarnations. Every other boss in Super either has a weak point and obvious strategy, or is a push over. Ridley is an erratic beast that can be hurt anywhere but his tail. Dodging skills are a must, and even the screw attack can be defeated by his tail. It's a war of attrition, arguably the only one in the game, too.
I believe this game and the comic and/or manga that came out around the same time is also what started the whole personal issue Samus has with Ridley. He's the true threat in her eyes (much more so if you take into account the interpretation of the story where he is the supreme leader that rebuilds the pirates' ranks after every defeat and Mother Brain is just the AI base commander/lead researcher). After Ridley's defeat, getting the metroid larva back is the final part of her quest.