This is a good article with lots of research and observation
http://www.forbes.com/sites/laurashin/2015/12/28/should-you-invest-in-bitcoin-10-arguments-against-as-of-december-2015/#2715e4857a0b4841a9f54669
1. The parameters of the currency could be changed.
2. There could be an attack on the network.
3. Transaction volume moves off the Bitcoin blockchain into side chains or permissioned chains.
4. The system does not successfully transition from being subsidized by the block reward to being paid for by transaction fees.
5. The Chinese firewall or another Internet issue causes the global Bitcoin network to be split for a few days.
6. The lack of a central authority prevents the protocol from progressing.
7. A competing protocol could overtake it.
8. World events prompt a crackdown.
9. Government regulations in one part of the world put the whole network at risk.
10. People get burned by it as an investment.
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Among these concerns, 4,5 and 6 should be the most relevant currently, and it seems they are all related to the block size limit debate
It is interesting to see the author put the possible change of money supply as the biggest threat, I guess that's also the reason that the money supply is prohibit from being changed in bitcoin wiki
