My take on it after reading and playing the Gameday event is that if you were avoiding 4E because of what they did to the classes, particularly the martial ones, this may be the game for you.
The mage and warpriest definitely "feel" different and have far more options than the knight, slayer and thief. As an example, none of the martial classes have burst attacks, so multi-attack is exclusively the domain of the spellcasters (with the exception of cleave)--at least as far as I saw. Similarly, movement effects are much less common for martial characters, the occasional push or very minor slide.
Martial Essentials characters will be spending most of their time making basic melee attacks, and can do quite a bit of damage with them. I suspect players who want to do a lot of HP damage and not much else will love them. A slayer fighter is a good class for someone who's new to the game and just learning the ropes: they're kind of fire and forget. The slayer in the game I played in was handled by a younger player who had never seen 4E before and he attacked it with gusto.
All of that taken together means you didn't like the "sameness" of 4E classes, will probably be a very good thing. Essentials classes are quite different from each other based on their power source, that is for sure.
From my perspective, I'd happily play an Essentials game, but I would only play a spellcaster, since these new martial classes don't offer me the sort of thing I enjoy as a player. If the goal with the game is to draw in people who didn't like 4E, I think it's going to be pretty successful, providing they actually give it a look.
Just my thoughts...
--Steve