Learning the mechanics is fun. Using the mechanics in increasingly complex puzzles is fun. Understanding how the mechanics would still apply after they add in additional wrinkles is fun.
I will agree the Marsh was probably the most well done area in terms of staying fresh. I guess I just see the mechanics as a set of instructions we are learning how to use. Adding small wrinkles doesn't make it more complex because they only add one wrinkle at a time. There is a linear progression through the puzzles, so much so, that the new wrinkles and how to solve them are immediately apparent. Adding an element from another puzzle section is just adding another instruction. I don't see it as being more complex, just another instruction to complete the puzzle, like cooking from a recipe. I did enjoy the town, where it was a smattering of everything, but it changed enough from puzzle to puzzle to stay interesting. I hope the mountain is a lot like that.
I struggled most with finding out the individual concepts and some of the better wrinkles like on Symmetry Island where the entire rules changed. Those were the most fun, that discovery of new rules, not repeating puzzles with tweaks to rules. Then you have levels like The Jungle or The Desert and I just want to skip it because I know the rule, I know what to do, but I don't want to spend the next hour
finding exactly where to stand or listening to a chirp sequence over and over to map it out
It'll just be a fundamental disagreement I have with most people, and that's OK. I still enjoy the game overall, but sometimes I really don't enjoy the repetition (wrinkles and all) because the wrinkles don't really make it harder for me and I'm not sure I enjoy the base puzzle mechanic. I just think the game would have been much better had they focused on more of the discovery mechanics. I'd take 5 new mechanics and 200 less puzzles if the development time had to go somewhere.