Finished this last night with 100%. Such a great game and huge Kudos to Mercury Steam for nailing not only a competent entry, but an entry that easily sits in the top tier of 2d Metroid games imo.
My largest complaint regarding the experience is probably the large scale boss encounters and pacing. While the smaller metroid encounters gradually ramp up into tougher versions it wasn't until the
boss that I really thought the encounter design was starting to touch on the levels of some of Prime's bosses in terms of ability use and incorporation throughout the fight. It's always nice when Samus' tool set is given thoughtful and creative consideration. After that the remaining large scale bosses came quite quick and were a bit of a let down overall. There isn't much to say other than they just weren't as creative or nearly as fun.
I agree each area not having it's own iconic theme, distinct color palette or even a name hurts the game's ability to give weight and character to the separated zones. However, there is cohesion within each area as one section transitions to another sharing small components to signify a gradual exchange in environment. I also think there is such a large variation of backgrounds and scene setting at play within each area, that you often feel that the area itself loses a bit of a larger identity. Maybe, more signature audio tracks for each area consistently present, despite whatever atmosphere the background elements and design were setting, would go a long way towards creating a sense of identity for each area.
On a parting note, I'll only say that the hardware form factor really hampered my overall enjoyment and I'll certainly levy those complaints separate from the software itself. Coming in and out of morph ball with the thumb slider was often clumsy and the ability to use the touch screen wasn't really a great alternative as you have to relinquish control to do so. I think one of the reasons it took me so long to complete the game, was that I couldn't play for extended periods without thumb/hand cramps or even fingers going numb. This was all with a Circle Pad Pro in place to help give some extra heft to the unit.
Anyhow, I absolutely loved the game overall and I'm glad we have a new 2d metroid to enjoy (Prime is still my favorite followed by Super)!