Metroid on the NES is the game that started the Metroid series back then in 1986 and then was remade in 2004 with the game Metroid: Zero Mission on the GBA. I played the original NES version that comes as unlockable with Metroid: Zero Mission. Changes to the original NES version include graphics that are being a little squished to fit the GBAs lower screen resolution and the ability to save the password, so that one doesn't have to reenter it. To get a little closer to the original experience I tried to play it without referencing a walkthrough and instead draw my own map of the game.
Graphically Metroid definitively shows it age and even for a NES game it is rather basic, featuring no backgrounds and almost monochrome foreground tiles. The engine is also unable to handle multidimensional scrolling, so you are limited to horizontal- and vertical scrolling sectors, which are separated by doors. Different sectors do contain different looks, so there is at least a bit of variety to be had. One big disadvantage of this game however has is that it is full of recycled rooms, there are literally sections where each of four door ends you in the exact same looking room. This makes it basically impossible to navigate the game without a map, as you can't really tell where you are with so many rooms not only looking similar, but being pixel exact copies of each other.
Sound and music to a good job at creating the atmosphere and already feature the tunes that where recycled and remixed in later Metroid games.
The game features the same open world exploration as later games in the series, but it does not feature an automatic map system or hints on where to go. In this game you are basically completely on your own without any help. The gameplay is also noticeably different, most importantly in that you can't shoot down and in that you can't duck. These limitations allow enemies that are half the size that you can't easily hit with your normal weapon, forcing you to either use the morph ball bomb or the environment to properly align your shoot. This makes the combat a good bit more interesting then in later games, as you have to figure out proper ways to work around enemies and can't just shoot everything on screen with ease. The small sprites help to give you an overview over a large game space. The weapon upgrades then do a very good job in lifting some of the limitations. The ice beam allows you to freeze enemies in place, making it easier to dodge fast moving enemies and removing a need to fight them, while the wave beam allows you to shoot enemies that are half your size. Unlike in later Metroids the weapon upgrades don't stack, so you can have either the wave beam or the ice beam, but not a wavey ice beam. In the end fight against Mother Brain this lack of stacking forces you to go back to recollect the ice beam. Another thing mission from later games is the bomb jump, placing a bomb on the ground, then standing up, catapults you into the air, allowing you to perform a higher then regular jump. While the game does locks you out of some areas when you haven't collected a specific item, this is used much less then in later games, allowing you to basically explore the whole map once you have the bomb and missile, which you get early in the game.
Collecting weapon upgrades and suit upgrades in this game is as important as it is in the later games, but unlike later games, here the items are placed much less logically. A lot of items and areas are hidden behind seemingly normal walls that you have to blow away, giving you little to no hint to do so. The game even goes as far as hidden secret passages behind lava, which you normally try to avoid, but here you have to jump into to get critical items.
This basically brought my playthrough without a walkthrough to halt, as it is very easy to miss an important item, which in turn makes the game incredible frustrating. In my case I ended up defeating Kraid and then being in Ridley's lair, but missed screw attack, Varia suit and the wave beam along the way. What made the situation worse is how the password system works in Metroid works. The game doesn't feature save rooms, but instead gives you a password whenever you die. This password saves the sector you are in, your suit upgrades, your missile count and your energy tanks. What that password however doesn't save is the energy count, meaning you always start out with 30 health, even when you have health tanks that would give room for 500 health points. This means that in the later parts your character can take two hits before dieing. You can collect health points just as in later games by destroying enemies, but those enemies that are in location where you can easily harvest only give you 5 health points, making refilling your health an incredible tedious thing.
Being stuck in Ridley's lair, I chose to peak at a walkthrough instead of trying to refill up my energy manually. Knowing the locations of the new upgrades made things a lot easier, as it turned aimless exploration into a targeted treasure hunt. With the new items collected the game also become a hell of lot easier, as now not only you got twice the health from enemies, but the weapon upgrades also allowed to basically clean each screen, giving you much more opportunity to collect health. I'd go so far as to say that the game got a little to easy, as it went from basically 'impossible' to 'easily manageable'.
Kraid, Ridley and Mother Brain are the only boss fights in the game. Unlike in later game both Kraid and Ridley are here barely larger then a normal enemy, instead of screen filling. Kraid turned out to be rather tricky, not following an easy dodgeable pattern, but having full health helped to defeat him without to much trouble. Ridley on the other side was trivial, just as in all the other Metroid you could just spam him with missiles and be done with it. The projectiles he shoot were trivial to dodge. Mother Brain didn't provide all that much challenge either, as there only a few slow moving projectiles floating around, no fast targeted attacks. After the last fight against Mother Brain Metroid typical countdown follows, but it is rather large and the escape path is very short.
Overall this is still a very fun game after all those years, playing it without a walkthrough however turned out to be not the best way to enjoy the game. The same looking rooms make the navigation by memory extremely hard and even with a hand drawn map, it is just to easy to miss critical items, turning the game into an near impossible to solve chore. With a walkthrough on the other side the experience is completely different. You no longer need to just randomly walk through same looking room, instead you focus more on your next goal, this also leads to a steady stream of new upgrades, thus making the game much easier.
That you only start with 30 health points, instead with full energy is probably the by far biggest fault of the game, as it adds a lot of unnecessary tedium and renders the energy containers mostly useless, as you almost never will find the time or motivation to actually fill them early in the game. The base difficulty of the game on the other side is very welcome, as it forces you to be careful and figure out proper strategies and patterns against enemies. Early in the game you are also forced to dodge enemies instead of defeating them, something not seen in many games these days.
PS: Source code for the NES version of Metroid is floating around on the net, it is not the original code, but a disassembled ROM that is heavily commented.