I want to start this post by saying that I absolutely love, love, Knytt Underground. I blazed through it in a couple of days and enjoyed every moment. It feels absolutely wonderful to be back in the world of Knytt/WaDF - one of the most beautiful and enthralling worlds I've ever had the pleasure of exploring! Anyhow, onto my criticisms of the game!
Art
KU is an undeniably beautiful and atmospheric game, and it's nice to return to the world that Within a Deep Forest created for us, but I think it's lost a little of the distinct character that each area of WaDF had. It probably helps that WaDF was divided into discrete levels, but even in Knytt and Knytt Stories it was very clear when you were travelling from one area to the next without the transition seeming jarring. Probably the one thing that contributes to this the most, and I know I'm by far not the first to point this out, is the choice to have a completely black tileset for the ground. It worked really well for Night Sky because of the whole silhouette aesthetic, but I'm not sure how I feel about it in KU. It makes the entire world feel quite uniform and the areas, while clearly quite different in terms of atmosphere, blend into each other a lot more and the result is that far fewer of them stick in the memory long after playing. Another contributing factor is the fact that the entire game is set underground, so while there are a number of large caverns you will be looking at tunnels for a large majority of the game which means there's less variety in terms of screen design, which again means the areas lose some of their distinctiveness.
Another point, and this is a personal opinion which is heavily influenced by nostalgia, is that I really miss the pixel art of the earlier Knytt games! I like Nifflas's new photo-based art style and it certainly offers something unique in the sea of pixelly indie titles but part of me (the nostalgic part) hopes he hasn't abandoned pixel art for good!
Writing/plot
This is the first Nifflas game to feature a significant amount of dialogue and anything resembling an actual story, and he's done a pretty good job of it considering. I very much enjoyed the world building and the character development, but simultaneously I was very frustrated by the lack of resolution to any of the plot points raised throughout the game.
Full story spoilers follow
Okay so Nifflas created this wonderful world for us to explore filled with colourful characters to interact with and interesting factions, none of whom are completely agreeable nor completely disagreeable (a triumph in the video game world of good/evil dichotomies!). You're sent on some generic video game quest to ring five bells and save the world from destruction, but of course that's not the point of the story - in fact it's heavily disputed as to whether the world is going to end at all. The point (as Nifflas's in-game avatar points out at the end) is the journey rather than the destination. What bothers me is that you encounter a whole host of conflicts along the way and hardly any of them are resolved in any satisfactory way.
You meet the Family, who are claiming ownership of land and then forcing people to pay them money unfairly. There was an option to poison and kill them which may have offered a resolution but I decided I didn't want to. Turns out there is no other way to deal with them (which honestly is probably quite realistic).
You meet the green blobby people (can't remember their proper names) who are native to the caves. You learn of their history and how they had a war with the sprites that they lost horribly. Your fairies wish that they could find a way to get the sprites and the blobby people to live openly in harmony, but the blobby people object and insist that their location remains a secret, so it just goes unresolved.
You learn of Cilia's broken relationship with her parents but are offered no opportunities to patch it up. You're also introduced to her tragedy of a love life but again, no real resolutions there either. (Note that I had Cilia do almost all of the talking for my playthrough, so I probably missed some of Dora's character development).
You're told about the great artifact, a giant clock which the Myriadists insist is going to end the world unless you complete your quest. Of course they won't let the Internets, or indeed anyone else, take a look at it to see what it actually is (probably just a clock) so you never get to find out. In all fairness I don't mind this one because the ambiguity surrounding truth in religion is one of the themes of the game (as encapsulated in the encounter with the mage who tries to teach you magic - you never get to found out if it actually works because Cilia refuses to try. She thinks it's all hocus, but then she was wrong about the spectral diamond so what does she know?).
I feel like there are so many plot threads like this in the game - an interesting premise or conflict which is never followed up to a satisfying conclusion.
So those are my thoughts on the game's flaws. What do you guys think?
P.S. As an aside I really hope that Nifflas gives us another chapter or revisits the world in some way. I found the hints regarding the ruined surface extremely tantalising, especially when you find the teleporter and realise that you're right below Pinewood Heights!