For me, It wasn't the sparse background in the original Metroid so much as the bullshit NES difficulty from enemies coming at you through doors when you're defenseless etc. The lack of save points you mentioned was definitely a big factor too.
I'd like a remake of Metroid II as well. It seems like the odd duck of the series, not to the degree of, but maybe in the similar vein of Zelda II? I only ever played it on nintendo8.com or some other online streaming website though, so don't quote me on that.
As for Super, honestly, looking back I don't think I would have cried at the ending of it like I remember EGM staffers saying they did, but you're right; it's a shame not to at least experience that ending without knowing about it. I got stuck on a wall jumping section in my emulating days. I would always pass the game up at Blockbuster because Ridley being on the cover had me thinking "dragon=JRPG=genre I suck at." 
I got Fusion and Prime at around the same time, since they released at the same time and I found myself playing Fusion more because it was easier for me to get into. Like you, I had never played a Metroid before and I'd get stuck in Prime when the hints were too slow in coming, so Fusion's linearity helped ease me into the series. I honestly liked the story too, so I didn't mind the dialog the first time.
I still need to get a used Gamecube controller on Amazon so I can beat Prime on hard after all these years. It still looks good to this day.
Zero Mission was a bit on the short side for me and the boss battles felt like wars of attrition, but I'll certainly take that over the original Metroid's NES-hard design. I've completed it many times as well and it really is an awesome example of how to remake a game. I love the added stealth sequence at the end too. I think more games should be remade from that era with just the hint of added story and cutscenes Zero Mission succeeded with. It's a blast to play on the Game Boy Player.
I also need to finally buy and sit down to finish Prime 2. I didn't like having to deal with the damn dark world bubbles either, as I felt I needed to pause several times to think about where to go next so I didn't suddenly lose my forcefield while planning. It might not have been that bad if they didn't also limit your ammo.
I only played the demo of Prime Hunters and had fun with it. I planned to buy it despite the ugly-N64 look of the demo, but then the review hit.
I only ever played Prime 3 at a GameStop and thought the motion controls were cool (and new at the time), but my only other knowledge of it is the Retsupurae guys' Let's Play of it, which I recall being funny. Pretty sure they did the whole Prime trilogy.
Similarly, I've only seen the Other M "movie" ... as given the MST3K treatment by Retsupurae. And damn. Her relationship with Adam was just insulting. I still would like to play the actual game though.
