I've only played the Beta, so I'm sure my impressions are out of date now.
It's good to see that massing dryads isn't such a dominant tactic any more. That used to tick me off because it's not the way they're meant to be used, but after a while, that was the only thing Night Elf players did. And it usually worked, too.
This should be pretty fun at the start when people are still figuring things out. During the beta, I used to rip up Orc players who kept trying to mass shamans against me while I was playing Humans. It was great.
My favorite hero is the Shadow Hunter for Orc. One time, I was playing with my roommate as an ally. He was Human and I was Orc. Big Bad Voodoo + Holy Light on the Shaman = destroyed enemies.
Serpent Wards are pretty cool, too.
I hope they've balanced out some of the neutral heroes, though. Some of them were completely ridiculous such as the Naga Sea Witch with Forked Lightning.
My one complaint during the beta was that air power got seriously shafted. There's almost no reason to use flying units any more. There are too many commonly used units that can rip them up (spellcasters and ranged units). The point of air units is to attack other air units (especially Frost Wyrms and Chimeras) or ground units that can't hit back, but if the enemy is always going to be able to hit back just be following a normal strategy, what's the point?