One problem that I have with Bitcoin is its complete dependence on the strength of its crypto. While breaking it may seem impossible at the moment, advances in computation speed and cryptanalysis may one day lead to it being broken. This would basically make Bitcoins worthless overnight.
Of course traditional currencies can be forged as well, but printing huge amounts of forged bills and bringing them into circulation involves all sorts of logistical challenges that don't apply in a digital world. Moreover, forged bills are typically not absolutely perfect and can be distinguished from genuine money, this is not possible with a purely digital currency. The fact that paper currencies can be forged is incorporated into the design of the system, whereas the Bitcoin system is based on Bitcoin's crypto being unbreakable.
It's not just speed, but number of nodes. If someone could, lets say in 10 years, build a block-chain of equal length to then-current one, they would still need to control > 50% of the nodes a significant portion of the network sees.