Great post. I disagree with Jeff about stickying this for now, but I think this is a good springing board for serious discussion about custom levels and their role as entertainment vs art and what makes a level good. At the end of your post, you say The fun factor of a level is the most important thing. I agree with this and will explain why, but I also want to provide a slightly different take on fun factor: it being not a separate component from creativity or presentation, but a conglomeration of many different components flavored by visuals and uniqueness.
It's important to discuss what makes a level good and whether good is objective, but first let me relate excellence of a level in Marble Blast to excellence in other media, like film, music, and literature.
I think excellence is more objective in games such as Marble Blast than in other media. These other forms of entertainment/art have many purposes. Some movies are meant purely to entertain, like your average summer blockbuster, while others are meant to challenge our viewpoints. Some albums are meant to be catchy. Others are meant to display technical prowess and enjoyment might come from appreciation of the intricacy and how perfectly the instruments all fit together rather than enjoyment on the first listen. Some books are meant to be page-turner novels, while others are meant to satirize popular culture, or be innovative in the use of allusions or style such as the poem The Wasteland. Some food is meant to taste really sweet and fatty like ice cream while some is meant to combine different spices, textures and colors that may not be instantly as delicious as mac and cheese or a greasy hamburger but takes a refined palate to notice and appreciate nuances in flavors. That said, all food has to taste good. All movies have to capture one's attention. But, especially with movies and music, from person to person, what is enjoyable is very subjective. I could make an argument for works of art from both categories, entertainment-focused or art-focused being equally good, but the reality is that I want to experience both on different occasions, in different moods. Sometimes I like to escape into a (maybe cheesy) fantasy world in a movie--say Toy Story, while othertimes I enjoy the feat of figuring out a movie--say Primer or Memento.
A comedy might have the same rating as a slow-yet-rewarding drama, for example: one form of film isn't necessarily superior. However, if a film can combine entertainment and technical prowess (let's say an amazing score, writing, acting, and a unique plot)--that's when critics rave. But a movie that entertains one person might bore another to tears, and that's why a lot of what reviewers hone in on is, did the artist achieve what his purpose was?
In Marble Blast, that purpose is to entertain. Nobody buys Marble Blast so they can compare the idea and execution of the levels. Being a racing/platformer, that means most of the time the fun comes from traversing the course in a way that might encompass going fast, using powerups, and/or navigating hazards. Enjoyment doesn't come from appreciation of technical prowess: that isn't what marble blasters play the game for. Movie-goers might watch a movie like Upstream Color to appreciate the innovative style of filmmaking at the expense of a logical plot, but in Marble Blast the plot is everything. I agree with Buzz that Fun Factor is exactly as it seems - the quality of a level is how much you enjoyed playing it. But it's important to discuss what goes into that, since there are other things that factor into fun factor than the challenges themselves.
One of those components is originality. Originality has two divisions, the first is a measure of the uniqueness of the level as a concept; the second I'll discuss next. For example, is it a half-pipes level, a beat-the-clock race, is it a climb-to-the-top level, a fortress level, is it an open multiplayer-style map? Does the level have a memorable theme to it, like Marblaxia's Hideout? In my opinion, originality is definitely a component of the enjoyability/fun factor of a level: it determines whether the level leaves a lasting impression in my mind. I read about a psychology experiment a while ago that concluded adding a few minutes of wind-down time significantly increased patient's positive impression of a surgery. The group tested that did not have the wind-down time remembered a painful surgery, while those of the other group whose last few minutes of the surgery didn't hurt were much more pleased with the operation. Last impressions are huge. In custom levels, a unique overall feel of the level leaves a positive lasting impression that colors your overall opinion of the level.
The second level of originality, let's just call it creativity, relates to the actual construction of the level. It is how unique the actual challenges of the level are constructed. A climb-the-tower level with rolling, jumping, slopes, frictions, and powerups is not creative. Add some launchers to get to the next platform 100000 units above and maybe a little more. Make it so once you get to the top, you have to flip gravity and work back up and you're getting there.
Creativity **does not by necessity** make a level better. Creative challenges aren't by nature more fun. I think this gets back to your idea that gyrocopter challenges are trite but no-one minds some of it in a level because while not anything new, flying over obstacles and across gaps is pure fun. A creative course might make a level better on paper, better as a form of art, but getting back to the purpose of Marble Blast, it is not necessarily entertaining to play. You don't want to go back and play the level again to go for a better time, just since the challenges are unique. Replayability is the key term here and I think it is the most visible face of a good level.
Level building for most players doesn't play into fun factor and subsequently affect how good a level is. That said, things like misalignments can negatively affect gameplay, and things like stretched-out platforms may bother players. And more importantly, the presentation of a level is huge. Properly-plated food can subconsciously affect how much someone enjoys a dish at a restaurant. A variety of colors on the dish does the same. While the flavor of the dish might be the same, these visual cues affect our taste of the food.. the brain really is extraordinary. Presentation of Marble Blast levels is the same. Color scheme, scenery, and the skybox may not affect gameplay but they do affect fun factor. These affect our enjoyment in the same way that originality does: we as humans are stimulated by complexity. Marble Blast isn't all about controlling rolling, it's also about painting a pleasant scene. We prefer Pablo-style slopes with rounded bottoms and tops than flat slopes because they are more complex, we prefer wavy paths over straight rectangles and platforms with trim and sitting on top of a huge mbu_pattern_cool2 wall over untrimmed floating courses. Visual complexity subconsciously determines in part our enjoyment of a level even before Go sounds.
Take it to far and you have style over substance, however. The best level combines a fun course with a visually-stimulating interior. A flat-course of rectangular platforms can be fun if it allows for pathfinding, but an impressively-built, overly-technical level that does not let you go fast (as you mentioned, Buzz), improve your time (say, a course built around a moving platform where the goal is to stay on it through a hazard-filled map but your time is basically when the MP hits the finish) isn't a level you want to play again. So, style and substance both affect replayability, but I think style adds to substance, and not the other way around. The best-looking pie isn't one you order again if you don't like the taste.
Going back to creativity, I think it's style. It elevates a level that is already fun. It stimulates us with figuring out new ways to complete a level and makes us appreciate the innovativeness. But unless the creativity is also intrinsically fun, it's merely garnish.