Here's my take on the Slayer, based on what we've seen so far:
It feels like a pocket class.
What I mean by that is, I feel like they found an unexplored niche in the Martial options: a two-handed weapon Striker. And they wanted to do something with that. But as they developed it, they quickly found that this particular niche was very niche-like: you couldn't really add much depth to it without ending up with a character that overlapped the Fighter's builds.
So they ended up sitting on it. But once they started working on Essentials, they looked at it again, and they realized it would work just fine as a Fighter build that was, more or less, entirely self-contained. Suddenly you could be a Martial Striker that specialized in two-handed weapons. But it's called a Fighter, because it's not robust enough to stand on its own.
I also find myself wondering whether, with roles not longer being fixed for classes, if we couldn't get rid of classes altogether. It's not a tremendous jump; the biggest difference would be that you'd pick a power source and role, and then have a selection of builds that are available. From that point, you'd grab from a universal array of powers that are inherent to that power source. On their own they'd be fairly bland, but depending on your role, they'd be tweaked. And of course, a small number would be unique to given role/power source combos.