Serialization is one of them http://cr.openjdk.java.net/~briangoetz/amber/serialization.h...

I think the most interesting one to me was an interview with Joshua Bloch in the book Coders at Work. It was to do with generics, of course, but distinct from the usual surface-level debate over type erasure. He felt that generics were rushed, and that they could have done a better job of sorting out the details of how things were going to work at the language level if they had given themselves more time.

Wild mixes of consumer and provider API's for example in Swing. The later designs have been much better in that regard.

The built-in http client library is borderline horrible.