Security Vulnerabilities and bugs
It's possible but unlikely that a newly discovered bug or security vulnerability in the standard client could lead to a block chain split, or the need for every node to upgrade in a short time period. For example, a single malformed message tailored to exploit a specific vulnerability, when spread from node to node, could cause the whole network to shutdown in a few hours. Bugs that break user anonymity, on the contrary, have been found, since the pseudo-anonymity property of Bitcoin has been analyzed less. Starting from version 0.7.0, Bitcoin client can be considered a mature project. The security critical sections of the source code are updated less and less frequently and those parts have been reviewed by many computer security experts. Also Bitcoin Satoshi client has passed the test of being on-line for more than 3 years, without a single vulnerability being exploited in the wild. See Common Vulnerabilities and Exposures for a detailed list of vulnerabilities detected and fixed.
Energy Consumption
Energy consumption for mining has a high correlation with bitcoin value (exchange rate). Because variable costs of mining are dominated by electricity price, the economic equilibrium for the mining rate is reached when global electricity costs for mining approximate the value of mining reward plus transaction fees.
So the higher the value of one bitcoin, the higher the value of mining rewards and transaction fees, the higher the energy consumption of the bitcoin network in the long run.
more efficient mining gear does not reduce energy use of the bitcoin network. It will only raise the network difficulty
cheaper energy linearly increases mining energy use of the bitcoin network
the same conclusions apply to all proof of work based currencies.
