Protoss:

Stalker: While somewhat fragile, Blink more than compensates by allowing you to keep them out of harm's way or sniping key enemy units.

Sentry: A great low-tech spellcaster capable of significantly changing the odds if used well. And Hallucination allows for a variety of uses.

High Templar: Still the reason for my screen name. Psi Storm damage has been nerfed, but the clumping of units still make them lethally effective, and Feedback is a great complementary ability, much better than Brood War Hallucinate.

Dark Templar: Somewhat disappointing. The high investment needed to get them, the inability to easily switch over to High templar as in Brood War, and the general abundance of detection makes them far less valuable than they used to be.

Phase Prism: Giving the Shuttle greater variety of use is a huge plus in my book.

Mothership: Not really as powerful as I would wish for it to be, given the ridiculous investment necessary to obtain one.

Terran:

Marauder: I don't like this unit at all. If it's tough enough to survive a psionic storm, it doesn't count as Terran Bio in my book. Besides, it lacks a genuine weakness - It has high damage, high hit points, good mobility, a slowing attack that makes it incredibly potent against small groups, Stim Packs which makes them incredibly potent period, and they can be healed by medivacs. I would have preferred that they reduce their hit points to 65-70 and their supply cost to 1, make them more aligned with the general theme of Terran Bio which was cheap, highly mobile, high damage output, but vulnerable to splash effects.

Ghost: Incredible upgrade to the unit. Not only is it now lower tech, but it has good damage output, easier to obtain Nuclear Missiles, and its EMP and Snipe abilities are much more invaluable than the old Lockdown. Like the Marauder though, it should have less hit points. Long range and cloaking should be defence enough considering it's a bio spellcaster - High Templar and Infestors have low hit points too.

Reaper: Recent games have shown these guys to be ridiculously good against Zerg when used properly. They somewhat remind me of Mutalisk harass in Brood War.

Zerg:

Queen: They finally made this unit not only useable, but genuinely indispensable. Its role fits perfectly with established Zerg lore in which the Cerebrates delegated control over battlefield units to the Overlords and Hive control to the Queens, all three of its abilities have strong value, and its defensive fighting abilities are useful to defend against certain rushes. With the addition of the Zerg Creep mechanics, the new Queen may be the greatest success of StarCraft II.

Zergling: These units have been seriously crippled. Adrenal Glands needs to be buffed big time, Zerglings quickly become inadequate right now.

Roach: This units has trouble fitting in. It was too powerful when it cost 1 supply, and is too weak now that it costs 2. I would have liked to see if follow its original purpose, be a low hit points but high armour/high regeneration units, making it a great tank against low damage burst units like Marines/Zerglings/Zealots but vulnerable to powerful damage blasts from Marauders or Immortals or Dark Templar and the like. By reducing it's power versus those units via specialisation, they probably could have kept the Roach at 1 supply while keeping the game balanced. Though it might have been bad for ZvZ.

Corrupter: I really wanted to see this unit have a melee attack. It could have caught and slowed down enemy ships or flyers with its tentacles then rammed it or bit at it with its beak. It just doesn't inspire much in its current form.

Infestor: This little unit has a lot of potential in it, I think.

Ultralisk: The Ultralisk transitioned from a huge tank it Brood War into a massive damage dealer in StarCraft II. And I love it.