The post doesn't actually tell you much about the nutrition without evidence about levels of actual nutrient absorption, but a quick google scholar search finds that calcium inhibits iron and copper absorption, especially in the presence of phytates [0,1] (which Soylent contains from the oat flour) and that zinc, manganese, and iron have similar uptake pathways and limit each others absorption by competing for use of those pathways [1,2]. No idea if there's any other interactions, but it doesn't seem like a good idea to rely on this stuff much for nutrition.
Aside from that, he vetted the formula by an uncontrolled, unblinded study with a sample size of n=1 (i.e. trying it himself for a few months), and he says on his blog that he "started having joint pain and found [he] fit the symptoms of a sulfur deficiency. This makes perfect sense as [he] consume[s] almost none, and sulfur is a component of every living cell. Sulfur is hard to miss in a typical diet so the FDA would have little reason to recommend it. A typical male physique has 140g of sulfur, making it the sixth most abundant element in the human body. Ten grams of sulfur from Methylsulfonylmethane cured me right away, and I now consume 2g/day." [3]
In that same post, he says that "After three months I should be finding deficiencies, and I did", and makes major revisions to his formula, apparently assuming that he's figured it all out this time. He puts a significant amount of fiber in the formula for the first time, going from 1.2 grams to 40 grams, in line with the 38 grams recommended for men under 50 [4], which was probably a good idea considering how much low fiber intake increases the risk of heart attack [5].
A month later, he responds to criticism by saying that "there have been no deficiency symptoms, and if this becomes a problem the amounts can be changed to compensate" [6]. His dismissal of concern ignores the fact that many known nutrient deficiencies can take much longer to manifest, and have severe irreversible harm (like B12 deficiency leading to nerve damage, or developing heart disease after years of insufficient fiber intake, or calcium deficiencies and osteoporosis). He also says that "The initial sample size was small and the timeframe short, but the results are easily reproducible, as shown by the community site discourse.soylent.me", as if having a greater amount of data overcomes the fact that the sample was extremely biased, still short-running, and with nothing even resembling a credible study design or consistent data collection.
Maybe it's not a bad thing that Soylent's probably not going to take off?
[0] http://www.ncbi.nlm.nih.gov/pubmed/1600930
[1] http://journals.cambridge.org/download.php?file=%2FBJN%2FBJN...
[2] http://www.ncbi.nlm.nih.gov/pubmed/2058577
[3] http://robrhinehart.com/?p=570
[4] http://www.mayoclinic.com/health/fiber/NU00033