I believe that bitcoin still needs a "killer app" in terms of something that can be done with it that can't currently be done with other transaction methods. The most likely of these in my mind appear to be:
1) low-fee and quick international wire transfers
2) SMS money transfer or barcode money transfer
3) online gambling from countries where it is restricted
4) silk-road type marketplace
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Once bitcoin opens the door for an activity many people are already engaged in (or want to be engaged in) it will likely drive all sorts of innovation. Personally, my hope is for #1 and #2 to be combined, as I think there are benefits to getting Western Union out of the international payment business. However, a large gambling site like Bodog doing #3 would likely drive an incredible uptick in BTC usage as well.
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(Edit: hmm perhaps this doesn't answer the question. Of the above I also think that #4 is the big one (though I think a good client/wallet app will also include near-instantaneous transactions and also offer wallet security) but I think that these problems, even if surmounted, will not offer much in the way of growth until one particular niche really takes off)
Humorously I was writing a question that may solve #1 as you were writing this: http://bitcoin.stackexchange.com/questions/382/what-percentage-of-bitcoin-transactions-are-orphaned
– David Perry – 2011-09-01T17:54:51.890As asked this is too subjective to be answered effectively and will only invite speculation. – eMansipater – 2011-09-01T23:00:46.020
1Some questions can only be answered speculatively, yet answers are needed because people who work on these weaknesses need to prioritize. Reasoned, plausible speculation is good if it's the best we can do to solve an actual problem. – David Schwartz – 2011-09-02T04:39:56.807
I think the scope of the question is too large for this format. – Amin – 2011-09-02T08:42:04.227
@David in real life, yes. On Stack Exchange, no. If it can only be answered speculatively, there is no way to clearly distinguish "correct" answers through votes, so it's not appropriate to the way this site works. How exactly is one supposed to identify which are "probably most serious"? There's no possible criteria by which to measure this other than to guess. Stack Exchange is not for guesses. – eMansipater – 2011-09-02T15:12:31.730
Voting to reopen with "What are the known limitations / weaknesses of BitCoin" ? – Pacerier – 2012-06-18T03:02:13.680
One problem is that "the user is responsible for his own security". Combine that with "security is tough" and we have a real big problem. – Pacerier – 2012-06-18T03:19:50.827