It has a lot more backtracking, and areas that all look the same, so there's no sense of progression when you play until you get to the Sanctuary. There's only so much brown and purple a person can take before it becomes damn near impossible to tell anything apart or remember where anything is, and that's bad in a game whose primary method of keeping players motivated is the possibility that exploration will reveal new and interesting things. When you know you're just going to find another goddamn purple cave it's hard to pretend that you're having fun exploring any more.
It also doesn't help that it's the first Metroid game where getting new abilities doesn't really feel special. Metroid, Metroid II and Super Metroid all added a ton of upgrades and felt different enough that even the same upgrades offered something new. Metroid Prime was basically Super Metroid for upgrades, but putting it in 3D was enough to make old toys feel new again as well. But in Prime 2, you just get what you had in Prime again for the most part, and since the actual mechanics feel identical anybody who played the first Prime just has a massive sense of deja vu. It's hard to care about getting new items when you know exactly what it'll be like to use them before you've even booted the game up.
Basically, Echoes shows everything it has to offer within the first hour or so, especially if you've played the first game. It's extremely polished and technically well done in any given segment, but it all comes together in the worst possible way.

