Raicuparta said it QUITE well. I suppose I'll just add to what he said/paraphrase. I wouldn't call Knytt Stories an "art game," but instead, a medium for art. It's similar to a canvas. It doesn't mean it has to beautiful--heck, it doesn't even have to have any semblance of "meaning" whatsoever.
Allow me to organize each primary element of a Knytt Story:
Each of the following four elements are almost unavoidable in Knytt Stories, and you won't find many levels that cut them out completely.
Level Design: While this might not always be a primary concern (a very simple, linear level design can go a LONG way if the other elements are covered well enough), it is generally what drives the challenge of a level, and, no matter how you split it, the level design of every single level out there (save for perhaps a very select few) is entirely unique as a whole. This might not be what defines your level, but it's easily one of your greatest tools in making it your own (and really, it's the only one you absolutely cannot avoid doing yourself, unless of course you have others doing the level design).
Visuals: Backgrounds and tilesets excel in setting the tone for your level. They also can be a big element in your level design; I'm currently working on a small level in which fog is used to obscure objects and parts of the level. Custom tilesets and backgrounds are generally always accepted, if not encouraged, here. We like new stuff!
Sound: Also generally extremely important in setting the tone for your level. I myself prefer to create my own music to throw into my levels, and, though this mostly wasn't the case for Will's Grand Scheme (my first KS level), it still included only music that wasn't a part of Nifflas' stock set. I'm a sound person, so I know I personally enjoy hearing music outside of the stock set, but... well, it's still pretty good music, anyway. Regardless, if I hear custom music that I like, I'll generally be hooked. Now that I think of it, though, sound, too, can be a major player in level design, with the use of sound cues and the like.
Story: Well, it is called Knytt Stories, is it not? Story has the largest scale for being unique, as you can go from having an incredibly elaborate story to having absolutely no story at all. Still, it will usually rely on the two prior elements to really engage the "reader."
But, in general, if you express yourself and put it out there, someone's bound to like it. The more polished or unique it is, the more people's attentions you're likely to garner.