It's not "in some ways dated", it's massively dated in almost every way. The design/exploration don't hold up at all - it's an absolute nightmare to navigate that horrible mess of a world without a map, especially when most rooms are identical except for the occasional random hidden block you can bomb to get through to... another copy-paste room, most of the time. The atmosphere maybe holds up, okay, but I wouldn't say the music does, honestly. There are a few good songs in the game (Title and Kraid, mostly), but also some terrible ones (Ridley and Tourian), and I've never been a big fan of the classic Brinstar theme (from a compositional point of view, it's so simple and barebones it hurts). It's not the game's worst aspect, but it pales in comparison to almost every other Metroid soundtrack in my opinion, and I'm glad its composer never returned for future games - his style just didn't fit the series.

The worst part is definitely the difficulty, though - not because it's hard, but because it's unbelievably, unacceptably cheap. Starting with 30 health regardless of how many energy tanks you have is stupid enough, and then made worse by how insanely long it takes to farm for energy, often resulting in farming sessions that can take upwards of 15 minutes every god damn time you die just to get to full health again - never mind having to travel all the way back to where you were before dying, through that confusing mess of a map. Then there's stuff like getting hit by enemies as you go through doors (this is even unavoidable on one specific door!), not being able to hit any small enemies without using bombs (which isn't necessarily hard, but it's unbearably inconvenient), or getting stuck in acid until you die because you couldn't tell which was the fake acid between two identical looking rooms. And don't get me started on the bosses... Ridley is a piece of cake, but a terribly designed one; but Kraid and Mother Brain, well, if you don't know exactly what you should be doing from the first second and do everything perfectly, you're basically guaranteed to die within seconds. And then get respawned halfway across the map, with 30 health. Lol. Yeah, I'm never playing that crap again, thank you.

It's great how much it innovated for its time, but looking back and knowing Castlevania released on the very same year, it just puts in perspective how cheap games like Metroid were even back then.

In other topics, I really should get playing those GBA Castlevania games.

To me the first Metroid is the best and it’s still playable (I’ve played it quiet a bit on the NES app you get with Nintendo Online). Super Metroid is a superb game by all means, it’s just that the first one is better (to me).

It comes down to the ”no hand holding” approach. No minimap, no markers (for items or objectives), you are just dumped into the world and need to find your way. Playing it today I can finish it fairly quick because I roughly know where I need to go to. But for a first playtrough you get some serious time out of it (unless you use a guide or look things up online).

While Metroid Prime is an excellent series (if you exclude the DS title) I don’t really consider them as Metroid, except in name. The 3D environment messes up the Metroid feeling.

Well, I won't address your opinion because it's a hard disagree here, but you really should give Hollow Knight a shot. Feels like it's right up your alley.

I love Metroidvania games and it's been such a blessing that they have been going strong in the last few years because for a while it seemed like a dead genre.

Some of my favorites are:

Super Metroid
Castlevania: Symphony of the Night
Guacamelee 1 & 2
Hollow Knight
Ori and the Blind Forest
Axiom Verge

Hollow Knight is on my ”to play” list. 

Oh I also wanted to add that although I love the Prime series to death, I don't consider any 3D game to be in the Metroidvania genre. It has to be 2D.

I don’t want to offend anyone but I hated the original Metroid, Metroid zero mission is amazing though, better than fusion and almost as good as super.
To me symphony of the night is still the best, but today we have many great metroidvanias, one game that doesn’t get enough attention is shatae and the pirates curse, that game is really fun and does metroidvanias in it’s own way.

My very first Metroidvania was Blaster Master. And then maybe Faxanadu (if it can be counted...maybe not). I didn't play the original Metroid until a number of years later.

But Jesus H Christ, I love the Metroidvania genre. It scratches my gaming itch so perfectly every time. Oh, and I do consider the first two Prime games to be Metroidvanias. Sure, it's 3D, but the underlying exploration and progression is classic Metroidvania. The third game is a little different since it's got multiple planets.

Yeah it’s just a great formula. Some mechanics and gameplay loops just lend themselves to better experiences and the Metroidvania formula is a great example of that.

I’ve actually been working on a top 10 Metroidvania games—which will probably be top 25 when it’s all said and done. There’s been an explosion in the sub-genre lately, with 2018 being a banner year.

my first metroidvania was fusion. it might be blasphemy but its still my favorite metroid, although i thought zero mission was pretty amazing too. the new generation of metroidvanias has been top tier though. ori and the blind forest, guacamelee 1 & 2, hollow knight, axiom verge, and even teslagrad (which is kinda linear) were all great games that i replay whenever i don't know what to do with life