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
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.
(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)