Really when the Beta is released we will probably still be another few months away, since a lot of the final balancing is done during the Beta process. I still look back to when I beta tested StarCraft 1 (Top 10 Beta Ladder, WOOT!) and how over the top unbalanced some things were.
Vultures could THROW spider-mines, and the mines did damage to buildings. You could even target the building and throw the mines and they would run over nice and happy to explode on it. Took a small group of vultures just a split second to take out a command center, nexus, or hatchery. It was not long after that they made it so the mines could only be placed a short distance and had a "warm-up" time, and couldn't damage buildings.
All the caster characters, like the High Templar, Defiler, Queen, Science Vessel, etc... all had regular attacks. The Queen actually had the attack that is right now used by the Mutalisk. Queens were horrible to fight due to that attack mixed with the power of Broodlings. They ended up making the caster style units more squishy by removing the attacks and shifting a few of them around. A few of the single player character kept them though, like Tassadar uses the old High Templar attack.
You would think that since the Queen used to have the Mutalisk attack, that the Mutalisk had a pretty horrible attack, right? Not so. The Mutalisk used to have an acid spray attack (You can still see them use it in the final cinematic in which Tassadar kills the Overmind.), and it was so overpowered that a zerg player spamming nothing but Mutalisks and Queens would win the game pretty much every time. You could have dozens of Goliaths, Wraiths, Carriers, etc and it wouldn't matter, zerg would kill them all before losing anything of note. When they moved the Queen attack over to the Mutalisk, it balanced out that force a lot by making the attack a little weaker, even though the Mutalisk is still the backbone of any zerg flight force.
That is pretty much what the Beta was about, finding those units that people abuse and knocking them down a bit before retail hits the shelves. The bugs and such will also be squished, but I know Blizzard is pretty good when it comes to bugs with their QA department, so I am pretty sure bugs will be very few during the beta period. I only saw one bug during my playtime at BlizzCon, and it happened in the Cantina during the little adventure portion of the Single Player, the game itself felt flawless.