I enjoyed MP:E while I was playing it, but I recently sold it back; I don't think I'll ever really go back and play it again. MP on the other hand, hells yeah.
I found MP:E to be an ultimately frustrating experience not necessarily for the difficulty level of the enemies, bosses, what have you, but because of the mechanics.
- The Dark World was tedious; even when you got the Dark Suit.
- Ammo for the different guns forced you to sit around and either conserve your ammo for the bosses or specific puzzles/doors, or have to kill enemies just to collect it back.
- The fact that there were maybe 3 novel items in the entire game (and the Screw Attack seems Retro threw in just to shut up the 2D Metroid-ies) and the rest were the same as MP
- The fact that you even needed to re-collect said repeated items, even though MP:E is a direct continuation of MP, and that you started the game with all your stuff anyways
- The keys felt like something out a Rare game. So did the artifacts in MP, but by the time you got around to those, you were such a bad ass that it was kinda fun exploring all those crevices you left behind.
That being said, I liked Echoes quite a bit; just not as much as MP. I thought some of the environments were fantastic (the Sanctuary Fortress) and the bosses were awesome (Quadraxis was great.)
I feel for MP:3, they need to just start Samus with nothing. Have her be caught off guard, with absolutely nothing. Also, whoever said that she should be able to combine her abilities like in SM is absolutely right. Also... please... add new items to play with. MP was so great because there were so many implementations of old items in 3D, but so many new ones to play with.
That being said, I think MP:E is like the Majora's Mask of the Metroid Franchise. It's a fun, enjoyable game, but ultimately a frustrating experience in the end (and I'm replaying Majora's Mask so I could go off on that one too.

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