First of all, a bit of sidestory about myself:
I'm German. Therefore i apologize in advance for any possible mistakes or misunderstandings my review could contain.

I named this posting "Feedback" (especially for the developers) and not "Review", since a normal review can be found on my
homepage.
This posting therefore especially focuses on personal experience & interpretation as well technical stuff regarding the ingame mechanics. Note that i'll always try to be as constructive as possible. I loved playing Aquaria and finished the game yesterday with roughly 75% of all hidden extras found - and rest assured that i
will continue to explore as much remaining content as possible.

I've been playing games since i had a C64 and i LOVE oldskool looking games to death. Aquaria was really a big experience and should be mentioned with other legends like Zelda, Metroid, Cave Story in one single breath. You people did a great job there and i hope i can enjoy your next game as much as i enjoyed this one.

But now - on with the Feedback
GraphicsAquaria had something, that many other games lack: Atmosphere. I really enjoyed the vivid and colourful world design, the only flaw i could come up with was a small lack of "emotions" on the Actors. More facial expressions could've helped to get a better impression of Li's and Naija's feelings. Same goes for the bosses, who all have an "enrage" mode but don't really show it in their face.
I'll dare to compare this with Cave Story which also had rather static actors but used small character images to convey emotions. Altough there was no voiceacting in the game, the pictures helped loads to get a better hold of the current scene happenings and character emotions.
However, the (red or blue) screen color effect and fitting sound / music /voice helped to convey the mood pretty good. Overall i see this only as a minor flaw in Aquaria. Also I understand that integrating voice-fitting facial expressions are a very time intensive and not-vital feature.
Sound & MusicAwesome! I can't really say that i didn't enjoy the soundscape of Aquaria at any given time. The music tracks were some of the best i've ever heard in games and don't have to hide behind any "big budget" score out there. The voice of Naija was beautifully done and covered up almost any lack of facial and gesture expression in the game.
The Aquaria main "tune" which was hidden in many of the other tracks was conveyed so well that i will remember the song long time after playing this game. I just can't wait for a soundtrack to come out.

Long story short: No criticism here
GameplayThis will be the biggie in my feedback post. My thoughts here were "ambivalent" during huge parts of the game.
Level designFor every detail i got to see and explore in this huge world, i was a bit annoyed by the extensive level design. There are some parts where you swim in really huge areas and (except for some enemies) nothing big happens. Every level had those parts, but especially the abyss, the stomach and the ruins (in the east part) had too much "no-load operation" parts with repetitive designs.
I'll also allow myself to compare with "Cave Story" here which had similar level sizes but more "demanding" jump n run or agility parts and riddles. You did a good job in integrating faster travelling into the game, but still i often had that feeling of "ah damn i need to swim over there again".
I know that Aquaria should be a more "calm" game and transport an impression of an extensive deep sea world. Still i thought that "less is more" in that matter.
Naija's DesignNaija's Character was conveyed pretty well, mostly due to the brilliant voice acting by Jenna Sharpe. I really enjoyed the whole integration of singing into the game which underlines the female character of Naija and also gives the player a more "mermaid'esque" impression of her. Regarding her character development (including the lost memories) i can say that i enjoyed playing and accepting her as my personal female hero.

The major flaw in Naija isn't exactly herself but the mechanic behind the usage of her 8 different forms. For every new form Naija gained i sometimes was a bit disappointed in its proper use. Energy, Fish and Beast all had good uses, but especially Nature and Light seem to be more of an "endeavored" kind and didn't really had much use except passing certain level-parts of the game. Furthermore the whole "change aspect" could've been emphazised more as Naija still remains too "humanoid" in most forms. I wouldn't say that i was bored with Naija's different forms, but disappointment about some of her form-abilites and usages was one of the biggest emotions i felt during the game.
Personally I'd rather have less forms to switch through and instead more powerups to get more use out of the existing forms. In this special example, energy form could've been easily upgraded with a light effect (resulting in less clicking during the Abyss part) as well as a multi-target feature since the normal "rapid fire" almost had the same effect. Beast form was already something close to nature and could've been upgraded with a grow-plants- feature.
I know that this is a very VERY subjective impression, but i think that most people don't want to remember 8 songs or constantly choose from 8 forms during a hard boss fight (which is already demanding enough when you don't know their weakness yet). Using the 1-8 Keys helped me loads, but still i had a lot of misclicks during a fight, resulting in damage (and annoyance) which i could've easily avoided.
Li's DesignAs much as i enjoyed the first encounter with Li i got disappointed by the ongoing game parts involving him. Except for the memories (which have to be found first - and i only have found 1 this far) there is very little storyline and character development on Li's side. Except for the "opening the door part" which showed his engineering skills i looked at Li more as of a "assist fire drone" like you have em in R-Type.
The ending part in the stomach however helped to a lot to show the bond between those two and i personally considered the last combined-form as a very cool twist in the story.

As a result: Li was a very interesting character but just got too little screen time to show his traits and skills.
CookingAs interesting as it was at the beginning i personally felt overwhelmed by the vast selection of recipes you get confronted with during the game. The "getting ingredients and cooking" mechanic fits the underwater setting well. But the menu itself is rather uncomfortable to use and also interrupts the fluid ingame happenings a lot (especially during boss fights where people usually take a load of damage if they don't know how to evade properly).
I also found myself looking at the recipes tab a lot which i personally found rather annoying. Therefore i reduced cooking to a minimum (usually when my ingredients-container was stuffed). If there were some sort of infight-quick access menu or some sort of "cook multiple leaf poultrice at once" button, i would've enjoyed that part a lot more.
Whew! That was more text than i initially had in mind to write.

I hope the developers read this and get a good use out of it. As said before, I intended to be construtive, so i hope no one feels offended by my writings. I enjoyed playing Aquaria a lot and i will continue to do so. I thought that those 13h of gaming were well spent time of my life and i'll try to gather as much storyline (and secrets) out of Aquaria as i can get.