If we keep in mind that MSE it's just an old fashion antivirus , then we can say that Microsoft has a pretty good antivirus.
However if we take a look at what Avast is doing with their new Avast 6 ... well Microsoft needs to work more on their Essentials.
MSE includes some cloud features but it is still signature based detection. Microsoft is forced to offer a product that everyone can use. It can't display pop-ups and it can't have many false-positives.