IOTA is the first distributed ledger protocol that goes 'beyond blockchain' through its core invention 'Tangle'.
The IOTA Tangle is a Directed Acyclic Graph which has no fees on transactions and no fixed limit on how many transactions can be confirmed per second in the network; instead, the throughput grows in conjunction with activity in the network; i.e., the more activity, the faster the network. Unlike typical blockchain architecture, in IOTA there is no separation of users and validators; rather, validation is an intrinsic property of using the ledger, thus avoiding centralization.
Next step in cryptocurrency is solving the balance between security, incentive and usability in a different manner. Although I am not technical enough to understand the newly proposed chains in-depth, there have been a lot of initiatives on this point. IOTA is one of them, and Byteball another.
However, it's not all about changing the consensus model, as Nimiq shows with it's focus on usability and (very) thin clients, although having a remarkable resemblance to Bitcoin.
It is encouraging to see these initiatives popping up. At some point, we will have enough hands on experience with these variants to determine whether they are going to be stayers or goners. Funny that I would wish hackers to attach a coin to test its security instead of remaining void of hackers. In that respect, Bitcoin seems to have passed the test of time with its simple and effective code.