Why is Terran so favored in top level play?
Terran is extremely versatile. In GSL, where you have to face many opponents with varying playstyles who have had a chance to study your previous games, you can easily be sniped. The best Terrans, like MVP, are capable of playing dozens of different "standard" styles effectively. On the other hand, the best Zergs and Protosses are still funneled into three or four basic "standard" strategies that don't allow much room for strategic innovation or trickery.
On ladder, you'll rarely see anyone enough to get a good grasp of their playstyle and how to go about countering it, but teams in GSL specifically prepare for days to match up against one opponent. Additionally, there has been talk that the 1-1-1 build is the root of the problem because it allows so much versatility to either expand or hit incredibly hard-to-stop pushes.
Finally, Boxer and Nada were two of the first to switch from professional BW to SC2. In Korea, their influence should not be underestimated- they are probably the two biggest names in competitive StarCraft ever and that's liable to cause a good number of up and coming players to favor Terran. This is most notable on the SlayerS team, which is made up primarily of a number of incredibly good Terran players, all new faces: MMA, Ganzi, Ryung, Taeja.
It's because Terran is OP!
Just kidding, seriously though, it is hard to answer this question because so much of it depends on the individual players. You have great terrans like MVP, Top, and Polt right now in the current Code S. But you still have players like July (still in it) that made it pretty far this season.
To understand the strength of the race you'd have to look at each match-up (trying to keep this short as this answer could be really long):
TvZ: Seige tanks and thors provide strong defense against a lot of the zerg army compositions. Mix in medivacs and marines to provide your attacking force and high DPS and you have an army that can roll over a zerg with early harassment. That said, if you let the zerg get away with their macro (like July) you will still lose.
TvP: Ghosts take away so much of the strength of a protoss army, an army whose strength is based on the use of energy and their life force comes from their shields. Tanks also make it difficult for any kind of strong push from the protoss deathball without taking a lot of early damage from the initial blasts.
That all said, each other race does have counters for the above strategies, dark templares, fungals, broodlords, etc.
It comes down to the mobility of the terran army. A medivac or two filled with a few marines and a few marauders can wreck your economy and/or tech tree, something that becomes really hard to come back from late in the game. This type of constant harass without your ability to break the terran seige lines allows them to get more ahead and, to use the term again, win the war of attrition.
The expansions have not been released yet(and it is the base game), so Terran has the good units already, as much as they are trying to balance it. However, I think the game is quite balanced.