Source: BlockGeeks, date indeterminate
In a distributed consensus-based on the proof of Work, miners need a lot of energy. One Bitcoin transaction required the same amount of electricity as powering 1.57 American households for one day (data from 2015).
And these energy costs are paid with fiat currencies, leading to a constant downward pressure on the digital currency value.
In a recent research, experts argued that bitcoin transactions may consume as much electricity as Denmark by 2020.
Developers are pretty worried about this problem, and the Ethereum community wants to exploit the proof of stake method for a more greener and cheaper distributed form of consensus.
Also, rewards for the creation of a new block are different: with Proof-of-Work, the miner may potentially own none of the digital currency he/she is mining.
Unlike the proof-of-Work, where the algorithm rewards miners who solve mathematical problems with the goal of validating transactions and creating new blocks, with the proof of stake, the creator of a new block is chosen in a deterministic way, depending on its wealth, also defined as stake.