Dustin Browder was hired by Blizzard based upon his work as game designer of Red Alert 2. If you look at the units in that game, the units in Wings of Liberty, and the units for Heart of the Swarm, you can kind of glean the philosophy he has in approaching unit design. He purposefully tries to create unique, gimmicky units that he feels are challenging to use and fulfills very specific niche roles. For Starcraft 2, it looks like he brainstormed a bunch of radical ideas that he thinks nobody has thought of before for units, and then he sticks them onto a bunch of units and distributes them among the races as he thinks is appropriate.
For a campy, crazy game like Red Alert 2, that kind of philosophy is perfect. The game's aim is to provide you with a wide variety of crazy and creative units to use, and Browder delivers that with a lot of fun but useless units like the chrono legionnaire, mind control units, armored blimp bombers, dolphins, giant squids, and the like.
Those capitalist pig-dogs in Japan can't see the true potential of tentacles
For a game like Starcraft 2, however, I don't think his approach is appropriate. Starcraft 2 isn't a gimmicky game, nor is it a pseudo RPG game like Warcraft 3. Starcraft 2 a chess match of large armies. Like chess, the pieces of the game should be simple in their nature yet complex in their interactions with other pieces.
Consider the simplicity of units in Brood War. Most units have very obvious strengths and weaknesses. High dps units have low health, low dps units have high health. Terran units that stray too far from supporting units like the siege tank or the medic are significantly weaker than Protoss and Zerg counterparts. In Wings of Liberty, these sharp differences have been significantly muddied by units with in-between roles such as the high health, high DPS marauder, the almost never used spellcaster Mothership, and the meaty roach which is almost like a zerg zealot with a ranged attack.
Do you remember this shit? TvZ and ZvT had fucking completely different skill sets with zero overlap
With Heart of the Swarm, the uniqueness of the individual races are even more muddied to the point where even though the units are different, the play style differences of all three races become moot. What is a Warhound but a marauder that can be made in a factory? Why does Terran need another meaty unit that can deal high damage to armored units? Why do the Protoss need another muta counter if the Phoenix was originally designed to BE the muta counter?
Basically, it would be much better if Browder focused less on trying to "shore up" the weaknesses of the races with fancy gimmicky units. Instead, he should design units that are more conventional but ballsier and play to the races' strengths. Make the weaknesses of each race even MORE vulnerable, but make the strengths unmatchable. Bring back the Beta siege tank with 60 splash damage but nerf the shit out of everything else, 40 hp marines and 60 hp marauders. Make Terran super dependent on defensive ranged firepower. Remove the Roach but make zergling and hydralisk-like units even cheaper and cost even less supply. By keeping units simple, you can greatly exaggerate the strengths and weaknesses of each race making them more dynamic, diverse, and exciting to play.
Remember this? This is the way TvZ was intended to be. Not a marauder in site.
Another flaw of Dustin Browder's unit design approach is that right now, NOBODY knows how the new units will perform, or how they will actually change gameplay. What everyone does know is that the weird justifications Browder and David Kim give for the units are all just bullshit and we won't know if the units are good or not until the pros start using it in GSL, MLG, IEM, and other tournaments at the highest level. What is the oracle supposed to do again? Nobody knows.
New units introduced in a highly-anticipated expansion pack is supposed to provide excitement and anticipation, not confound everybody who sees the unit. When Brood War was coming out and I saw the Lurker, or the Corsair, or the Medic, there were some extremely obvious potential uses for these units. They seemed simple, powerful, exciting, like units that make you go "oh! well of course these units should be in the game! I can see tons of things you can do with all of them". When I saw the HotS units, the only thing I thought was "well, I guess when I get the game I'll build a bunch of them and see what happens."
This simpler, more direct approach to unit design I'm proposing isn't anything new, but it's actually an established principle in many of the most popular competitively-played games. As examples, I can point out character designs in Street Fighter, gun designs in Halo and Counter-Strike, and the unit designs of Brood War.
1) Street Fighter
If you think about it, all characters in this fighting game have basically the same basic moves and a few specials. Even then, a lot of the specials are similar, like Ryu's Shoryuken and Sagat's Tiger Uppercut. It's just the different timings, ranges, and priorities of the moves that balance the game. None of this has changed much from the first iteration of Street Fighter 2 to the current Super Street Fighter 4. But Capcom is able to make 3482039 iterations of the same game work because the characters in the game, though simple, just feel nice. The simple execution of Ken's fierce Shoryuken, hearing three solid connect sounds, and seeing the opponent lit on fire creates a huge endorphin release that just makes you want the opponent to jump at you so you can repeat the move again. And again. It's simple, yet feels fucking amazing.
2) Halo
From Halo 1 to Halo 3, the weapons of the game haven't changed much at all. They're practically the same fucking weapons and same fucking gameplay. But these games make millions. Why? Because when you're holding that fucking awkward Xbox controller and wrestling the crosshairs and finally make that headshot with the sniper rifle against the twelve year old who's been calling you a faggot for the past 5 minutes, it feels fucking amazing. And you want him to keep calling you a faggot so you can say "you mad bro?" while you blow his head off a second, and a third, and a fourth time. The sharp crack of the rifle, the vapor trail, the simulated recoil, these are all very simple things that Bungie has tuned perfectly. So that each of the weapons, all of them completely generic and unimaginative, just feel good when you fire them and kill something.
3) Counter-Strike
And you thought Halo's space marine was boring and generic. Fuck, here you are land bound with a bunch of direct-fire, automatic, ballistic weapons that all shoot forward and can kill someone in 2 hits, body armor or not. No rocket jumps, no vehicles, no nothing. Yet people LOVE this game. Because the simplicity feels good. When the AK-47 roars in your earbuds and you see that low polygon representation of a person's head splatter red blood-shaped sprites everywhere, it feels amazing.
4) Brood War
Does anyone miss the roar of the Siege tank, the dull, powerful thud of a detonating scarab, the harsh screech of a mutalisk? Those are just a few of the visceral sounds toned down in Starcraft 2. Brood War, like Counter-Strike, relies a lot on the perfect sounds that effects make in the game that makes the units far more exciting than they actually are. I guarantee you, if Browder gave the SC2 zealot a manlier voice, removed the "we cannot hold" garbage, but left the stats the same, you WILL see a jump in zealot usage and people will think that the zealot seems more powerful. When you think back to the first time you see a zealot destroy a marine in only three hits while marines bullets bounced off its shields, it's really a wonder that TvP was actually a very balanced matchup.
What I'm trying to demonstrate is that simplicity can be infinitely more fun than complex gimmicky units that may or may not be useful. It actually provides the majority of the fun factor in some of the most popular games. In Starcraft 2, there some elements of the game that feel good: the TSSSSS sound of banelings crashing onto marines, the diarrheal squish overlords make when they pop, these are things that just look and feel amazing. But there are also tons of other units that are just completely mediocre: roaches with their flaccid attack sounds, marauders and stalkers attacking in general, the new mufflers installed on zealot psi blades and hydralisk spitting. I can't even remember what colossi lasers sound like, but I can still remember the distinct sound of a scarab detonation despite not having played Brood War in over a year.
As fun as Starcraft 2 is, it can be infinitely more fun and dynamic if the design principle behind the game focuses on the simpler aspects and not on introducing new, more complex units. Dustin Browder would do better if he thinks creatively inside the box rather than uncreatively outside of the box.
TL:DR HotS units don't feel good. In order for the game to succeed, shit should feel good.
EDIT: I apologize if the ending became more and more negative. The basic premises isn't that the new units of HotS are bad (since we simply don't know), but that they seem too contrived and gimmicky. I just think Dustin Browder's approach to designing Starcraft 2 units is not ideal for a game designed to be THE platform for e-sports over the next decade.
EDIT2: This post is not meant to be a pro-Brood War or anti-SC2 post, despite many of the comments posted below. The Brood War comparisons are simply a matter of familiarity and convenience.