I think we're comparing apples and oranges to trees and cows.*
I'm not sure we are... maybe apples and pears, but those *are* fairly similar. ;-)
Hm. I think, for me the degree to which I *don't* get bored of something, in the realm of aesthetics/design, is a measure of its success. Okay, let's phrase that in a less clunky and confusing way: the better - the more aesthetically accomplished - something is designed, the less likely am I to get bored of it.
Although that's misleading, too, because it makes it sound as if the tilesets ("what is designed") are the crucial element for me here, and they kind of... aren't. And being bored isn't really the issue for me, either. When I don't like the look of a level it's usually because it's not designed in an aesthetically competent way, and not because I'm bored... well, okay, if a level is just a long succession of screens with just flat ground and a few enemy critters on it, then yes, I will get bored, too. *g* Okay, now *I* am rambling...
Let me back up and approach from another direction: to me, tilesets are like... building blocks; brushes and colours; in other words: the material from which you build something. Categorically stating that any level built from a default tileset will be boring is, to me, a bit like stating that any painting that uses the colour blue will be boring because we've all seen the colour before. Or, if that is one level of abstraction too far to make the analogy work, it's like saying that any painting done in a particular style will be boring because the style is not entirely new.
Now, granted, there may be people who are so jaded that they only get moved by something entirely new and surprising and original anymore... but I don't think that that is the audience an artist should aim for (or rather, the artist should only aim for that audience if s/he feels the same way). - For me there is something (near-)universal about aesthetically successful work, and aesthetic success is not connected to novelty. You can do aesthetically successful work with the oldest, most unexciting materials. Maybe this has something to do with me being a craftsperson - we do tend to appreciate excellence of execution over originality of ideas (although I've had some fairly 'highbrow' training at a reknowned design school that did emphasise originality...)
(I think I'll bow out of this now... not because I'm angry or anything, but because I think I'm beginning to repeat myself, and I don't want to bore people here. Repeated arguments *are* boring. *g*)
... Ultimately, this may be just a fairly minor difference of taste, anyway: I don't think you're looking for total originality either: you want something that still feels like Knytt Stories, after all. You just like the 'subtle variation' I talked about in my previous post to happen in the tilesets, while I like to see it primarily in the screen design. It's not like (most) custom tilesets really add anything fundamentally new - just some new patterns, some new plants, etc.