At least at first to second level (that's all I have seen so far, no HotFL in Australia yet), the most noticeable difference is that the Slayer is incredibly simple to play. Simpler than the PHB1 fighter by far. The stances take the place of your usual at-wills like Cleave or Sure Strike. When you do your usual attack, it's a melee basic attack. When you get an opportunity attack, the same. Much less to remember and keep track of. No need to remember the difference between your at-will powers, attacks granted by Combat Superiority, and opportunity attacks like a regular fighter. (One gets a bonus equal to Wisdom mod to hit and the other doesn't. I always get confused between them.) With the slayer, it's the same for all. Melee basic attack plus stance bonus.
The Thief seems more mobile than the PHB1 builds, and separates out the extra movement from the standard action used for the attack. A Thief will use some kind of at-will move power then a melee basic attack, from what I can tell, whereas the Artful Dodger might use a standard move action or shift and then Deft Strike (move 2 squares before the attack). The result is somewhat similar, but achieved in a different way.
