For me personally, I ask myself this simple question: What made the first part of Echoes enjoyable, and what changed to make it unenjoyable in the second half?
Monotonous Complexity: I think a lot of the frustration came from my feeling that there was a lot of detraction by addition. It didnt need to be that complicated, and I get the sense that Retro was trying to go bigger and badder without realizing that approach may not always be a good thing. Prime struck a great balance between a big cohesive world and a navigable one. I was obviously fine in the beginning in Echoes, but as the game went on, it got less and less managable, until I just gave up and looked at the map all the time on where to go instead of intuitively learning the paths, something I do well in Super Metroid and in Prime.
I honestly think that nixing one third of the total rooms in the game (and the dark world) would have streamlined the experience immensely. Do I really need to run through 3 areas to get to an elevator? Or do a morph ball sequence to get to and from an energy controller area? What do you mean I cant get to the dark world room I want to from this portal?! Theres so much redundancy that it overwhelmed the areas that are supposed to hold my interest. And Im probably one of the few people that was underwhelmed by Sanctuary fortress. Ive always preferred the organic Metroid environments. Mechanical ones tend to be stale, sterile and repetitive, no matter how many fancy textures you throw on the walls.
Risk vs. Reward:I honestly cant believe what they did in the second half of the game. I would get a powerup and be totally eager to test it out on the last place I remember. On more than one occasion, I would access a new area only to be blocked again by another required powerup. I got nothing for my troubles, and would have to sit there and rethink where I needed to go again. Again, exercises in frustration through poor pacing.
Another issue is that the reward often does not justify the investment in time and effort. There are a lot of instances where its literally a ten minute puzzle and all I get at the end is a missile expansion. Another one where I had to endure probably 5 loading portals back and forth to get one powerup. But then an energy tank sits right in your path. Weirdness.
Keys: A lot of people bitch about the keys, and I think its justified. It doesn't help that you basically need the light suit in order to get anywhere on the quest, whereas in Prime, you can pretty much start picking stuff up half way through the game. The clues were not as retarded either. On a tangent: I know I'm in the minority in this, I really really hate the inventory system in Echoes.
Equipment: I dont even want to get into the equipment. Every piece of equipment in Prime 1 was used to great effect. Enemies had weaknesses to beams, visor use was strategic on more than one occasion and boss fight. I cant say that for Echoes. Why did they even put the annihilator beam in? To ping sounds and to open like two doors? Because thats all I used it for. The dark visor is a cheap limiting version of the x-ray visor, and the sonic visor is annoying to use, ugly to look at, and next to useless outside of one boss fight. (Final Dark Samus fight could have been using the Dark Visor with no difference in strategy)
Storyline: Many people already stated this, but the Luminoth/Ing saga are completely boring. It had potential, if the Luminoth werent such a bunch of idiots and the Ing had some sort of evil intelligence (it would have been cool to read their logs). Isnt it sad when the most entertaining bit were logs concerning the Space Pirates, who are pretty much non-existent in the storyline after the first few hours? Heck, most of the fun details are completely dropped in the second half. I also felt the Phazon portions were really awkwardly shoehorned in, but whatever.
Despite my rant, there were individual elements that really really impressed me. The music overall was great, many non-Ing creatures were absolutely stunning (the grenchler is probably my favorite beast in the Prime trilogy), and the boss fights were fun for the most part. But as a cohesive gaming experience, Echoes falls short, which is doubly disappointing given the pedigree of its predecessor. Hate to be harsh, but Im not sorry I gave up on the game 5 years ago.