The easiest Nintendo comparison to make between Metroids Prime 1 and 2 is that Prime 1 is like Ocarina of Time, and Prime 2 is like Majora's Mask. That comparison is really far from the truth though, because whereas Majora's Mask took the fundamentals of Ocarina of Time and turned them on their head in a really dark, interesting fashion, Echoes is still quite a lot like Prime, except much more difficult, and with new locales. It's that difficulty spike that people seem to have had some trouble with, as Prime 2 is really treated more as the Prime series' Mario Sunshine.
I'm here to say that people should stop comparing Prime 2 to other games in other series, and just realize that Prime 2 is the best interpretation of the darkness, isolation, fear and anxiety of the original Metroid in a 3D context. In that way, Echoes feels like everything that one would associate with the Metroid series, only taken to new extremes.
Make no mistake – the difficulty level of this game is off the charts, and for a recently released game (2005, I believe), there's only a handful of games that are this challenging in terms of both the gameplay and the mental faculties needed to complete the adventure.
In many ways, the first fifteen or so hours of this game feels unlike the first Prime. Gone are the easily distinguishable worlds that are Nintendo's speciality (the fire world, the snow world, etc. etc.) and in their place are much less easily definable areas. The swamp world? The rock world? It's true that they all blend in together, but it makes the world of Aether feel like a real, definable place, something that was true of the first game as well, although to a much more fantastical degree.
The game starts with a harrowing, dread-inducing prologue. Samus arrives on Aether only to find that all of the Galactic Federation soldiers who were dispatched there have been killed, and brought back to life by some dark force. This is how Samus gets entangled in the drama of the Luminoth, a peaceful race who have been nearly obliterated by the Ing, the other major civilization on the planet. In essence, then, Prime 2 is the story of a civil war, and Samus decides early on to help the Luminoth, mostly because the Ing are spreading a darkness across the planet, and because they're sadistic killers.
The dark world is the most obvious change from most other Metroid games, though they're old hat for Nintendo and specifically, A Link to the Past. What this means for gameplay is retraversing not only areas you've explored already, but areas you've explored in different contexts. The dark world is a twisted, nightmarish place, where even being exposed to the air slowly kills Samus. This means taking refuge in small areas of light and hoping that enemies don't show up. It's a terrifying thing, always balancing on the precipice of death, and it makes playing the game a tense, anxious affair. As well, it demands some serious mental faculties, as what happens in the light world has an effect on the dark world, and vice versa. This leads to some insanely difficult, far-reaching puzzles that require you to have, in essence, two entire worlds in the back of your head at all times.
The difficulty of the enemies has taken a sharp spike upwards compared to the original Prime as well, and a few of the bosses (especially on the Gamecube version) will undoubtedly give even the most "hardcore" gamer some serious conniptions. Add in the confusing, almost messy level designs, and you have a game that's even more difficult than the original Metroid. It takes serious perserverance, and the amount of time and energy you have to expend on this game could have some people saying it's not worth it.
Beating this game, though, was easily one of my greatest gaming achievements, and it's a game that's supremely rewarding to beat. This is mostly because the game relies on your intellect to beat it, and in that way it's not like trying to beat Ghouls and Ghosts or anything like that – it's a game that if you've played an FPS before, you should be able to grasp, but it'll be unlike any FPS you've ever played in terms of cerebral challenge.
I still don't think this game is as good as Primes 1 or 3, though, mainly because I feel as though the art direction took a hit in this game. It's murky and dark a lot of the time, and while that adds to the foreboding, it doesn't make for a very lively world. The purple of the dark world in particular is just a little too overdone, and while I can certainly appreciate the direction that Retro was trying to take this game in, it's clear that they realized their mistakes by the time they got to Prime 3, and opened up the claustrophobic world to great effect.
Still, for a genuinely "Metroid-y" experience, this is the 3D game to play. There's no denying that like its Prime brethren, it's one of the best games in its genre, and is really only overshadowed by its predecessor. (The less said about this game's multiplayer, though, the better. Not every game needs multiplayer, Nintendo.)