How Important is Scenery in Marble Blast?

Well, it depends what kind of level it is. In multiplayer, scenery isn't too important and the level can only work if the gameplay works. This is because of the way multiplayer functions. The satisfaction comes from basically nothing more than flying to diamonds, getting diamonds, and getting a good score. Since other people are in the game trying to beat you, getting diamonds (and maybe a few powerups here and there) is the only thing on your mind, and no amount of visual beauty or grand structural brilliance is gonna change that. Singleplayer levels, especially long ones, have a bit more to them. Fun can come from collecting diamonds and getting a good score (time), but also from speed, exploration, rhythm, threatening difficulty, aerial openness, puzzle solving, etc. And yes, all of these and more can be enhanced through scenery. For example, exploration can be enhanced if each section has a different color scheme or structural theme to make it unique and exciting. Flying can be enhanced if there are arrows around the marble that make the player feel accomplished for going with the plan rather than going through nothingness. Or it could be the other way around and a lack of scenery can make the player feel free. I still think that scenery is more enjoyable on a subconscious level, though. Don't get me wrong; scenery is very noticeable, but effective scenery doesn't necessarily shock the player and then get respect for how interesting it is, but rather enhances the experience as a whole and leaves the player feeling good toward the level.
But that's all scenery does, "enhance" the experience. If the gameplay is good, good scenery makes the level great. If the gameplay is horrible, good scenery won't quite make the level great. I think this all relates to the flow of the level. I'll put it this way: if nothing is annoying, if I don't have to replay a part over and over and get lucky, if I'm never dumbfounded at how difficult or tedious one section is compared to the rest of the level, the level probably flows nicely (a good example would be anything by Phil). If a level doesn't have that smooth uninterrupted feeling of classic Marble Blast greatness, then I start to think of the level as poor, whether or not it has circular scenery poles, detailed wooden curves, an interesting color pattern, or realistic textures. If a level, on the other hand, succeeds in keeping me delighted the whole time, scenery can make the level unique, memorable, grand, exciting, satisfying, or emotional. The thing is, immersion in a game comes from stepping into another world and forgetting that you're on your computer. If there's an annoying part of a level, whether it comes from luck, tedium, or otherwise, the player can get frustrated not at the world he/she's in, but rather at the mechanics of the game itself. If you're thinking too hard about what the game is doing to your valuable time, you're not immersed in the world of Marble Blast, and scenery won't change that.
On the other hand, even if your level already has the comfort and enjoyment of a classic Marble Blast level, what is it without a little atmosphere? What if you gave it a construction theme or a seashore retreat style or a magnificent ancient temple look? Or maybe you could go simple and give it some nice detailed wooden structures just to make it feel more official? These are things you can do to make your level more memorable, more magical, and more fun.
So here are my overall thoughts. In multiplayer, the look of the level is almost not important in the slightest, but in singleplayer, if a level is decent in its gameplay and doesn't annoy the player too much, scenery can go a really long way. And not just because it looks good in a practical sense or is impressive. It's more about the atmosphere, and I'm pretty sure we can all agree that atmosphere can make anything in any game go from decent to breathtaking.

So great to hear these tips of expertise from someone whose levels I have admired so much for a long time even before I decided to join the MB community. I hope to get started on constructor soon, and I really appreciate seeing these tips posted, which will inspire me since I have never used constructor before. I think a level that fits your descriptions of a magical, memorable, fun level, is your level the Orange Forest. In my opinion, it deserves a lot more credit than it has been given (nobody has made a vid of it yet). It has the perfect amount of consistent challenge, a nice rhythm, not too annoying, and when I saw the scenery, my jaw dropped. I love the atmosphere as if you're in a strange world where trees are orange, or in a forest in autumn. And the way you executed this theme turned out at an unrealistic level of beauty. Thanks again for starting this topic!