I'm not sure flavorlessness will make Soylent more popular. Most people like to have at least some flavor.
My point is that there already are a lot of products out there that are "fast, simple, and nutritionally complete that can be used from time to time as supplementation when you don't feel the need to sit down for an actual meal," but I don't see everyone who's so excited about Soylent already using them on a regular basis. OK, perhaps you would prefer to have something without sugar in it (I'm not sure why you would object to the other ingredients, fats and salts are essential parts of your diet), but I would think that if Soylent were going to be as popular as people make it out to be, there would already be lots of people replacing meals with existing meal replacements. All Soylent offers over the others is that it's bland, as far as I can tell.
I'm not saying the existing, well known ones are better. Just that they haven't revolutionized food or replaced traditional meals on a large scale or anything.
There are plenty of people who do already replace occasional meals with these existing meal replacers, along with energy bars and the like. So, Soylent will be one more out of a large number of choices in this field, produced by people with no particular food industry experience, and more bland than the rest. Is that supposed to get me excited about "the future of food"?
I can imagine people who already do use existing meal replacers, but having some particular problem with them that Soylent solves, might go for this. And then there's the Silicon Valley hype bubble, where a bunch of people will buy it and drink it for lunch for a month or two before getting bored and excited by the next big new thing (Twitter to your shoes! Personal Instagram quadcopter follow bot! Distributed decentralized FarmVille/GTA hybrid!)
The reason I'm so baffled by this is that it's hard to imagine why so many people would get excited about yet another product in an already crowded marketplace that's fairly niche (well, fairly niche for people who use them as their primary nutrition; relatively large for people who use it to replace an occasional meal). I don't see anything revolutionary about Soylent; it's yet another meal replacer.