Ethereum will switch to Proof Of Stake in some future hard fork called Serenity. For this putpose a new Proof Of Stake protocol called Casper is being developed.
However, there are quite a few cryptocurrencies out there that already use proof of stake, most of them a version called Delegated Proof Of Stake, some of them even adding a version to show how progressive they are (DPOS 2.0, DPOS 3.0).
What are key features of this Delegated Proof Of Stake as compared to current state of Casper and why is it not sufficient for Ethereum according to the Ethereum Foundation guys? Is it even secure enough in existing implementations?