There are generally two waves in which things get declared broken.
The initial wave is when tactics that seem powerful in theory simply are not that powerful in practice. Think back to the 3rd edition threads about Whirlwind + Cleave + Blind Kobolds. This will show up in about 3 months after launch.
The 2nd wave will be much more legitimate, as real holes are found in the rules. It took a while before the 3rd edition Haste spell was fully agreed to be broken. It was the one thing in the rules that revealed its self to be the one tactical choice that was NEVER wrong. This will show up about 1 year, but wont be fully agreed on until 2 years after launch until the pattern becomes prevalent.
One thing that will work in 4th editions favor is that 3rd edition gave the designers a few years of information about what kind of things are likely to result in broken and unbalanced rules. I think most things that evolved directly from 3rd edition are going to be quite solid, so no more things like a Tumble skill with no opposed roll / static DC's. I am pretty sure Save or Screwed spells are going to be largely reworked, as well, having certain limiters on them. So Hold Person / Fear / Hideous Laughter will have a per round mechanic that can allow escape. Spells like Power Word Kill, Finger of Death, and Disintegrate will be more like cloud kill, so that there is a cutoff point on Auto death keeping it from killing players when used at the appropriate levels.
I am however, more worried about things that are outright new additions to the game. We can be certain that at some point, the Bloodied mechanic will lead to some sort of exploit. I also think that something like the "Feather me Yon Oaf" mechanic will show up in the First wave of broken. Consider a team were every player uses bows and builds to have that ability. Broken on paper sure, but I do not think any exploit that requires everyone in the party follow a particular build will be that serious in practice. Another potential cause for a game balance break would be in the new racial abilities having an unforeseen synergy with a particular class and feat combination.
Finding something that is broken will eventually happen. I am also sure that something along the lines of "3rd Ed Druid being the strongest class ever" will be discovered. What I am less sure on is exactly how broken such an exploit will be. I think I will be content if the extent of the brokenness is marginal relative to the power of the other classes.
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