I'm a part of the beta. I play Protoss mainly and it's a helluva lot of fun (backdooring with Warp Prisms + Warp-in is sooo great) Stalkers are great for microing, Colossi are absolutely devastating.
The Mothership, however, is underwhelming. Immortals are amazing defensive units but have HUGE cost (250 min 100 gas) for the fact they are required to counter Roaches (Think Zerglings with range attack and 2 armor) and Marauders (Terran units that fire missles that do double damage to machines). Dark Templar are underwhelming as you have to build a Dark Spire that contains no tech upgrades and has no other purpose other than to build them, plus they have increased cost from the original game.
The Mothership is utterly amazing, although for the same reasons Arbiters were in the first game. The cloak mechanic combined with Colossi devastates everything when they don't have detectors and Recall is fun as ever. It doesn't feel like an "EPIC" unit, but it's hardly underpowered.
Carriers feel nerfed, mostly because they are utterly outclassed by Colossi with AA support. Phoenixes are the only Protoss counter to mutalisks, but that is also all they're good for, which kinda makes them useless. Void Rays are fun early, but so easy to take down in team battles they're really only good for quickly decimating unguarded expansions.
On the other races: Terrans usually go Marines/Marauders/Medivacs (think dropships with medic heal) and that's all they ever really have to do all game. Sure seige tanks are still nice, but Marauders do more damage to mechanical units than tanks in seige mode for the cost. Thors (Gigantic robots) are underpowered as they die really, REALLY fast to any decent sized army (2 Colossi would kill 2 Thors without one dying).
Zerg have an amazing early game, but suffer later as all they can really do is mass Mutas/Hydras due to the fact Ultralisks are too expensive currently for their utility, Infestors got recently changed to have a permanent Mind Control (Neural Parasite) akin to the Protoss Dark Archon MC from SC which in theory is awesome when Colossi or Thors are involved, but it's never happened in any game I've played because if it was vs Zerg, it was all determined by the rush.