Two very clear problems are that it introduces more complexity up front for beginners (because now to understand `length` you need to understand the concept of type classes) and the change had the possibility to break some extant code, due to it changing some type inference properties.

In practice, I think #2 was a fairly uncommon case, and the fixes were trivial and backwards compatible, so the cost was deemed pretty acceptable. #1 is a point of debate, because 'beginner' needs context (perhaps experienced programmers that are Haskell beginners would get over it quickly, but non-programmers would be substantially more confused, etc). Unfortunately I don't think we have a lot of truly empirical evidence on #1.