Recent environmental economic developments generate a need for sustainable and cost-effective (microbial) processes for the production of high-volume, low-priced bulk chemicals. As an example, n-butanol has, as a second-generation biofuel, beneficial characteristics compared to ethanol in liquid transportation fuel applications. The industrial revival of the classic n-butanol (ABE) fermentation requires process and strain engineering solutions for overcoming the main process limitations: product toxicity and low space–time yield. Reaction intensification on the biocatalyst, fermentation, and bioprocess level can be based on economic and ecologic evaluations using quantifiable constraints. This review describes the means of process intensification for biotechnological processes. A quantitative approach is then used for the comparison of the massive literature on n-butanol fermentation. A comprehensive literature study—including key fermentation performance parameters—is presented and the results are visualized using the window of operation methodology. The comparison allowed the identification of the key constraints, high cell densities, high strain stability, high specific production rate, cheap in situ product removal, high n-butanol tolerance, to operate in situ product removal efficiently, and cheap carbon source. It can thus be used as a guideline for the bioengineer during the combined biocatalyst, fermentation, and bioprocess development and intensification.