Guys, think about all the "badass" clone troopers in II and III. In II it was said that they age twice faster than normal human. So for clones it wasn't 20 years between III and IV, it was 40. Most of them were 20 "clone years" in Ep.II. Now add 40, we got 60, and that isn't a prime age for a soldier. So only young clones, made during Clone Wars, would become Stormproopers later. So there are two explanations to most Stormtroopers to be not as good: ether the new army of clones wasn't nearly as good as original, or most of them are not clones at all.

EU stated that the cloning facilities were shut down after cloners uprising, which means that only some of the Imperial Stortroopers are clones, while majority are regular humans, who aren't nearly as good.