I think one of the biggest issues is that languages such as Java account for large percentages of existing OOP in the wild and Java is a "kitchen sink" language with too many supported paradigms and styles.

For example, look at class attributes and methods. I'm not sure they're needed but in a language like Java the semantics often leads to wildly different programming styles within the same code base. And it's hard to enforce consistent styles using conventions.

That said, I've seen/read Kay asked directly, on more than one occasion to elaborate on his notion of a real OOP language implementation and his answers are always to coy and slippery for my taste.