1. The Viking
This unit is fairly versatile given it's ability to land on cliffs, and exploit certain advantages/disadvantages on certain maps. However, that's not really where the biggest issue lays.
Simply put, this unit dominates to skies too easily. Once a Terran masses enough of them it reaches a critical point where they're just impossible to beat, which absolutely forces you to focus on ground. This can be somewhat problematic in a number of ways, since siege tanks are currently dominating all ground units. See the problem? Sure it may be difficult to reach that point when you're Terran, but if you do the game is almost always in your pocket. Every air unit except vikings are bad against vikings. They're also inexpensive, which means you can mass produce them quite easily. They have insane attack range, which renders a lot of units useless with a bit of micro.
I really feel that Viking range should be reduced by a bit. I'm unsure what other balance changes could be done though since their HP seems fine, and their damage isn't that great until they're in higher numbers.
2. The Colossus
The Colossus can also be a bit of a problem when it comes to ground vs ground battles. Granted colossi are useless without their range upgrade, or sufficient AA support, but once they get it they completely dominate the ground unless up against a tank force. However with good support tanks shouldn't be much of a problem.
A colossus just does too much damage, and it's range is too great. I've lost a lot of battles due there being 1 or 2 colossi on the field under my opponents control. They may cost quite a bit and take up a lot of supply but for their damage output I'd say they more than make up for their price. This unit is literally every Zerg users nightmare, and it's still scary for Terran or Protoss. Bioballs get completely manhandled by a couple of them, and Protoss ground gets completely crushed by them too.
I can admit these units are really bad if they're by themselves though. As are most units though. A good player won't lose his colossi though.
3. The Voidray (The noobs friend)
Generally this unit is bad in small numbers, but that isn't always the case. If you don't scout it for whatever reason, voidrays can completely decimate your base and units in a matter of seconds. Once fully charged a voidray fleet is completely unstoppable unless you're REALLY prepared for a void rush, even then it's difficult depending on which race you are.
The problem is voidrays charge too easily by attacking buildings. Since they are flying units they can avoid any static defense you may have up and can get a free charge from one of your buildings. Then once you come to attack the voidrays, and they melt your army in an instant.
I just really dislike the fact that a handful of these can win someone a game if the other person is caught off guard. I think their charge rate for attacking buildings should be reduced.
4. The Ghost
This unit is pretty good in a multitude of situations due to it's abilities:
This is really a game breaking ability at the moment. Right now Protoss' hard-counter to tank mass, is Immortals. Which are completely dependent on their shields. One blast from a ghost can render them all useless and they become tank food. I've done it to people, and have had it done to me countless times. It's not just Immortals that suffer too, it's all units that rely on casting spells to be effective.
This ability isn't as bad as EMP however it's a devastating spell against Zerg. Zerg has no way to deal with it. Mutalisks become completely obsolete once a couple ghosts are out on the field. That's really my only gripe with snipe at the moment.
The only problem with nuke is it's very hard to see the painter. Maybe it's just me, or my settings. But I have a VERY hard time looking for the red dot. Especially in hectic games where I have more than 1-2 bases operating. It's very hard to find where the nuke is going to land.
5. The Baneling
Now before I make any points, I'd like to say that banelings are extremely useless in some situations. I am well aware of this.
Banelings are too good in ZvZ, in fact the matchup almost becomes a coinflip. ZvZ was pretty bad for this in SC:BW, but with the introduction of banelings ZvZ just became a whole lot more hectic and unpredictable. A few banelings can easily wipe out a large attack group of zerglings.
ZvZ has just become so much more one dimensional. It's generally just zerglings/banelings facing off, and whoever gets the lucky pop off wins the game. Sadly I'm unsure how this problem can be resolved. Roaches are a perfect counter to them, but roaches are very slow early game and can easily be out maneuvered. Leaving your mineral line at the mercy of the other player.