If you look at the nutritional info for Ensure[1] and Soylent[2] you'll see that Soylent has more calories, more fat, no cholesterol, more sodium, more potassium, less sugar, more protein.
Most important, Soylent has a more even distribution of the daily % of vitamins and minerals. If you drink 3 bottles of Ensure you're up to 180% daily value of Vitamin C, 240% of Vitamin D, 180% Manganese. The even distribution makes it easier for it to act as a complete meal replacement not just a diet or sports supplement.
My feeling is also that Soylent measures more and tweaks their formula more. Their goal is to be a good cheap meal for everyone:
Soylent is a food product (classified as a food, not a supplement, by the FDA) designed for use as a staple meal by all adults. Each serving of Soylent provides maximum nutrition with minimum effort.
In a paycheck or two I'll buy a bunch of these and they'll be my lunch and breakfast replacement. Things like Ensure don't cut it as a staple meal.
[1] https://ensure.com/nutrition-products/ensure-plus#vanilla [2] http://files.soylent.com/pdf/soylent-nutrition-facts-2-0-en....
