I'm no professional SC2 or SC analyzer, since I just play whenever I have time anyways and is never prepared for the 20-40 mins of stress in game. But I watch a ton of replays, and cast's by professional caster's and analyzers like HD, Husky, Day9, Artosis and etc.

I've followed SC2 ever since the begining, in Beta, didn't play it, because I was too lazy to go through the long arse procedure of downloading and blablabla, even though I had the ori CD key, and also my laptop at that time just sucked for SC2 Beta.

I got SC2 like almost a month after it came out because I was short on cash, and played only about 60-80 league games (forgot sorry).

So here's my thought:

Terren is often regarded as over powered because they're builds are easy, relatively cheaper, high mobility, easy combo's and ton's of tech to use. Their base unit's, the Marine can tech up twice with shields and stim pack and whatever it is I missed out upon. The Marauder, another relatively base unit is BUFF AS SHIT! and can tech up to slow down opponents and also stim pack up. Terren's also have a ton of creative units to use for harrassment like the Banshee's cloacked up, or Hellions, Reapers, bla bla bla.
There's tons of easy opening strategies to cheese with like Reaper harassment, early rush cloaked Banshee harassment, a rushed Hellion harassment, so on and so forth; and not forgetting the ease in mass producing drop ships with Reactor tech and mass marines.
The wide variety of tech and simplicity is one factor, another factor maybe the health of the units and high DPS (Damage Per Second). A 3M or MMM (Marine, Marauder n Medivac) ball is almost impossible to counter when they're rushed out, and you as a player hasn't teched up enough yet.
Terren's area able to drop scans, MULE's to harvest faster and more efficiently, repair injured machines, heal injured units and a whole F*@(#!G ton of other S*#T jsut because the can.

This doesn't mean Terren's aren't subjected to being un-cheesable or unbeatable though.
Terren Pros: Tech, straight forward units & Speed
Cons: Noob Terren, APM thats TOO low.
My recomendation: To be honest, I hate Terrens, just cause =D
Now I'll move on with Zerg, a race I loved and worshiped in SC:BW, when Teren's didn't have THAT MUCH S*@T in their arsenal.
They're highly speedy, easy to build, CHEAP and they really really require a lot less structural income, because all you need are a couple of cheap hatch's and you can create every unit after getting a single unit upgrade structure; eg: one spawning pool can make up to 9999999999999999 Zerglings.
The Overlords however are probably the single most annoying unit to have in my oppinion. They're units of high value because they can be used to scout, drop, and evolve to Overseer's to spot cloaked units, they determine your supply count and can poop creep to block or spawn hidden buildings in some hidden location. BUT THEY'RE SLOW AND WEAK AS HELL! An early marine push can wipe out all the overlords if not placed properly, and supplly block an opponent, and if the opponent hasn't macro-ed up enough yet or is broke, its an auto GG in almost every case I've seen so far. An early Stalker push by Protos can easily supply block a Zerg player as well, or even a Void Ray rush (Yes I'm in bronze, so I've used that kinda cheese before and I know what it does to the enemy =D).
This doens't mean that Zerg's lacking in tech though, this particular race has a counter for almost anything, but it's up to the players creativity. There are a couple of strategies and techniques professional Zerg players have used, even "FRUITDEALER" the current God of all Zerg followers; Baneling busts, burrowed Infestor's that will go all the way to an opponent base and spam out Infested Marines, 6pool (Spawning pool at 6 supply and rush, hurts the economy real bad though), Queen rush and the list can go on forever.
There is absolutely no limit to what a Zerg player can conjour up against any race, the same strategy can work against all, and Zerg macro battles are always awesome to watch just because of their sheer number of Zerglings.

Another thing that's fairly "imba" about the Zerg race is the creep, you're allowed to poop creep throughout the entire map possible, and it can be used sorta like a "map hack" or "auto scout" which is somewhat similair to a Terren's sensor tower. Only thing is, sensor towers are limited to a certain radius, but creep can spread all over as long as its on land! It also helps speed up units which is why smalller maps are often considered "ZERG FAVORED" just because its easier to spread creep and run your units over to the opponents base in an instant.

So why is Zerg less favored? Probably because players haven't invested enough time to explore the capabilities of every unit, and gave up halfway just because it is so vast.
But just look at how dear God, the Fruitdealer, obliterated all his opponents in the GSL with everything a Zerg has to offer! Apart form the standard expansion and all, he had differnet unit compositions almost every game, and went full on Ultralisk's only when the time calls for it, or just to end the game as quickly as possible. He's won games with purely banelings, like 140 of them (YA I'VE SEEN HIM DO THAT! HE'S INSANE), or purely Infestors, which don't even do any damage upfront at all. If any of you follow Husky Starcraft, you'd probably remember his Zergling and Queen rush supported by SPINE CRAWLERS! and only faltered because the opponent went  a couple of air units in the end.
To be a PRO GOSU TOP BESTEST GOODEST Zerg player, you'd probably have to spend alot of time and waste away a ton of hours experimenting on different scenarios. 
Pros: Cheap, Fast, Unlimited abliities if played right.
Cons: Lower HP, slow start, need's high APM and constant momentum.
My recomendation: Give it a go, Zerg's awesome, you need real high APM and constant focus on this one though, and tons of creativity. Zerg's a complex race and it ain't for the stupid no brainers to use.
I gave it a go furing my first 5 league games after watching Fruitsellers Zergling n Queen rush way before he won the GSL, but I gave up cause I was too lousy and lazy =D
Now for Protos, the current race I use.
Protos like Terren is RATHER straight forward as well, with buff arse base units, the Zealot, and ton's of tech to pick from, they're a super alien race after all. What made me pick Protos after I tried out Terren and Zerg was the easier builds and doesn't require that high a APM count. Like Zerg, their base unit structure, the Gateway is rather similair to a Hatch, one structure for all the units you really need to play a game. Protoss like the other two races in the game, has a "buff" unit, a air and ground attacking unit, caster's stealth, support and so on. Something I like about Protoss is that I don't have to macro my units as much as Terren or Zerg, because I have less support units in a battle; ie: no Medivac's or Queen''s or whatever. I can easily macro up, and rush a opponent knowing my base units will last longer than any other opponent base unit; it takes about 4 Marines / 4 Zerglings to take down a single Zealot.
Like Terren, Protoss units can tech up pretty high as well, but it's mroe of a no brainer compared to a Terren's because the variety of units in a Protoss army is considerably ALOT less than a Terren's. A "standard" macro push against a non-cheesing bronze league player may consist of just Zealots, Stalkers and maybe a couple of Sentry's, and that's be enough.
Less units mix = less use of brain = less stressful = less APM = more time for porno????? 
I dunno, but bottom line is Protos players are much more buffer and they look better too compared to the rest!
High Templars are the best counter to the 3m MMM ball too, with their insane cheap storms, which is also probably another reason why I picked this race.
However, to make up for all the "buffness" Protoss units take up more supply, as opposed to a Terren or Zergs; 1 supply = 1 marine, 1 supply = 2 Zerglings, TWO ZERGLINGS!! HOW CHEAP IS THAT! But 2 supply = 1 Zealot. Speed isn't really a Protoss strongpoint either, btu thats before teching up though. A simple reaper harass can wipe out an entire squardon of Zealots, just cause they move so slowly before charge tech. Collosus are SLOW AS er.. well, SLOW.
But who cares about reaper harass when the void rays come out anyways? =D

Pros: Efficient, high DPU (Damage per Unit), high tech, simplicity, easy micro.
Cons: Mobility, Supply requirements.
My reccomendation: this the easiest race to play if you hate Terren.
CONCLUSION NOTE:

You really cannot blame Terren's for being overpowred however, because Blizzard has spent over 10 years in balancing SC:BW with patches still coming out 10 years later, and is still stuck with the same work ethic and perfectionism with SC2. It's really up to a player's play style and creativity to win a game with any given race, so stop blaming the races!!!
Terren is PROBABLY slightly overpowered because the creators were human, and couldn't come up with enough idea's for units for other races?? =D Or they watched too many re-runs of Starship Troopers and any other movies that involves human's coming up with wacky stuff to kill aliens with.
It takes you a while to find your true calling as to what race you should use, so meddle around with the AI and training buddies more often, try out any scenario, think of counters and use every feature in the game to try and win.
Who knows probably by the next patch or expansion, they'd overpower other races just for fun, so that forumers and gamers like us have other things to whine about? But it's not like I'd ever use Terren though, I have self-respect.
END.
I KNOW I SUCK AT WRITING THIS RIGHT! BUT IT's 6AM! I JSUT FINISHED MY SLIDES FOR MY PRESENTATION THIS THURSDAY! I'M DOING THIS AS FAST AS I CAN!

Terran won't be OP forever. see coming patch 1.2