Calcium.
Studies have shown it blocks the absorption of Iron and Zinc. At least one even indicates that the combination of all three taken together can be quite detrimental[1]
I'm not a nutritionist, and I'm not even arguing against the possibility that a Soylent diet may be much healthier than the average American diet. I don't have a wealth of other examples where nutrients being taken together at the same time could be less than ideal.
But I do believe that these are questions that the people involved in Soylent should have concerned themselves with before even entertaining the idea of an all-purpose one-size-fits-all meal replacement. I haven't seen this addressed, and it boggles my mind that so many seemingly intelligent people are eager to pay for the opportunity to be guinea pigs of a meal replacement powder that could very well be harmful in the long run.
[1] (http://www.jstor.org/discover/10.2307/20486142?uid=3739960&u...)