While Security Essentials scored exceedingly well when it came to usability - 5.5 out of 6; which included false positives and system resource impact, but fell down really hard when it came to zero-day protection where it only picked up on 64% of new threats in comparison to the industry standard of 89%.
I'm pretty sure Microsoft's (questionable) monthly definition updates play a big role in this. Most other AV solutions update definitions as often as every day, hence such a high (89%) industry average which can in no way be entirely reliant on ever-failing heuristics: point in case, it's the sole category where MSE performs badly, just like it did on the previous test.