- "Numeric harmonization" which makes some of the existing issues with implicit numeric conversions even worse.

- Multiversal equality, which adds even more special rules to the implicit resolution algorithm, complicates the mental model of the language, is either extremely invasive or severely limited when applied to real-code, and falls apart when dealing with existing language features of Scala like variance.

- Addition of inline, which has absolutely no reason to be a new keyword.

- The tentative idea of adding T? for T|Null (as it appeared on some slides) which makes it obvious that no thought has been given to how the lessons of handling nulls apply to Scala.

Then we have enums, which are just poorly designed and executed, repeat the mistakes made with both case classes and scala.Enumeration, do not address the problems it is supposed to solve, fail to address actually valid existing problems, all while introducing not one, but two additional syntactic constructs which are unlike anything we had before.

On top of that we got incompatible additions in minor releases of 2.12:

- Additional places where commas can be added, which ignores one of the most common complaints about Scala: Too many syntactic variations to express the same thing.

- The addition of @showAsInfix which is a solution in search of a problem.

But at least unsound type projections got restricted, I guess. (That's the only major thing I can think of that will have a larger impact...)

Probably the removal of forSome and cleaning up existentials?