Creativity and making clever resourceful use of the level designer's functions do, in fact, stand alone from things that aren't merely "Gameplay" or "Aesthetically pleasing" or "graphics".
Remember Jumbo's level
A Goomba Tale, where he managed to make a full fledged, animated title screen of sorts by tricking out the level designer's capabilities (putting Mario under the stage out of camera sight yet still high enough to not go past the bottomless pit below, allowing a goomba to 'follow' him but make it look like it was moving to the right). This sort of thing was never done before AND it was well executed - it set the stage for the rest of the level really well. I'm not the only one to praise this either, just about every other judge for the 30th LDC gave points in "Other" for that, because not only was it something nobody had ever seen before, but it was just downright amazing that one could make a 'title screen' of sorts that set the stage for the level. If you were to award points for this kind of creativity, would you put it in Gameplay or Graphics? ... You wouldn't really put it in either, right?
And yes, Bugs and glitches could be noted as (un)intentional flaws in the gameplay, but they could also be referring to graphical glitches as well, which some designers have taken advantage of instead of letting it be a hindrance in the level. Item disabling - a noteworthy glitch used by many level designers to this date, is more geared towards the Graphical aspect of a level. I don't think that deserves to be in Gameplay, as obvious as that might sound, yet they are still notes about specific bugs/glitches used in the level regardless. I feel like the term 'Bugs/Glitches' encompass more and aren't restricted to just being gameplay elements.
If we want to be really technical, there could be instances in levels where disabled items/poorly layered graphics interfere with the gameplay (such as blocking visibility of the player/platforms ahead). But that could just as easily fit in "Other" as it could in "Gameplay".
If we're going to be stuffing tons of general criteria into bigger criteria points and making them so general to the point where they're called "Atmosphere", then I might as well just write all of my reviews under one criteria: "Pros and Cons". Both giving reviews and creating levels has so much more going for them than just placing down a few platforms and testing how good the 'gameplay' is, or how beautiful these 'graphics' look. There's much more to talk about in levels than these two subjects - Storyline (does anyone even remember the best part of MoD's famous level series Dark, or MP3 Amplifier's series Tearing Paper?), Replayability (If you ask me, I could go back and replay Buffooner's Shyguy Islands for the 20th LDC right now and still enjoy it), Flavor Text/Comedy (A positive level has worked wonders to keep the player engaged), Creativity (It's something I always try to strive for, though whether or not it leads me to being successful is another story), etc.
But that's obviously to not discredit Gameplay or Graphics at all! At heart, SM63 is a platformer - and the platformer genre is notorious for... well, platforming, fun, execution, and being simplistic in a sense. And that's not bad at all. But when you look at platformer titles you'll notice the big names tend to be unique. Super Mario Bros during its time was considered super unique, simplistic, fun, and very, very replayable. Super Mario 64 took Mario to a 3D world where the developers could do more with the game/series. LittleBigPlanet decided to let us be the creators and open our imagination to endless possibilities. FEZ took 2D puzzles to a whole new level with changing your perspective around what was actually a 3D world. I could go on, but... well, I'm sure you get it by now. Assuming enough heart is put into the level, any level made from anyone can stand out from the crowd, and be judged by something that is more than just 'gameplay' and 'graphics/atmosphere'. And while it may vary from judge to judge, I still believe there is something that we can still keep an eye out for.
Keep the Other category, if only for that.