Decision points. A PHB class might when they want to attack it have to make one big decision between two at wills, three encounters, and two dailies. All of which are partly understood - and this can lead to analysis paralysis. And that's in addition to choosing who to hit.
Meanwhile an Essentials martial has a binary decision as to whether to keep or change their stance (a simple binary choice), decide who to hit, and then decide whether to boost. Instead of one complex decision it's a lot of simple ones - and some people find this easier.