I'm well aware of the numerous possibilities that could lead to Bitcoin's demise. The most severe ones are fundamental technical problems, legal problems and the possibility of a new technology that renders Bitcoin obsolete. These are the big issues and challenges which we need to keep a close eye on.
It has also been proven that the issues don't have to be catastrophical for the exchange rate to crash. None of the bad stuff that happened in 2011 were catastrophical to Bitcoin itself, the issues were with 3rd parties. Certain important 3rd parties have a very important role in the economy which means that if they have big issues it can set Bitcoin back at least temporarily.
However I believe that the understanding of Bitcoin has significantly improved since May. And it's going to improve in the future. This will make the impact of 3rd party failures at least somewhat smaller. Even in this case we will always have to worry about the severe issues that I listed in the first paragraph.
One other thing, though. Even though the failures of 2011 did have a serious impact on Bitcoin, they did not outright wipe it off the map. This isn't a trivial point. Just imagine what would have happened with the US dollar if the New York Stock Exchange had been hacked and all stocks had been sold down to a few cents each... or the impact on the gold market if someone had stolen about 5 percent of the world supply of gold in any given two month period (putting together MyBitcoin, allinvain and Bitomat). The implications would have been far deeper and lasted far longer.
Will there be other hacks? Probably. Other thefts? Almost certainly. I can also give you an almost 100 percent certainty that there will be a crash of a large jetliner some time in the next two years, but that doesn't mean that commercial aviation is unsafe, and it sure as hell doesn't mean it's just as unsafe as it was in the 1930's, the 1960's or the 1980's.