this guy figured out one of the biggest problems with Bitcoin in the long term as related to the ability for anyone with decent programming knowledge to use the open source code to create their own cryptocurrency (litecoin and dogecoin are probably biggest 2 examples so far).
The Marginal Cost of Cryptocurrency
theres some advanced economic analysis in there, but the jist of it is that as new coins enter they will feature less transaction fees (because of declining rewards for mining and some other reasons) and become more attractive to users, which will ultimately devalue BTC. the theory is that BTCs value will only be equal to the marketing cost of starting a new cryptocurrency -- which is pretty low given its open source nature and distributed hosting model.
all that said -- i think there is still enough interest in bitcoin and media coverage that the value will increase before this long term economic effect is realized, so theres still opportunity for speculators in the shorter term. timing is everything.