Const or Immutable objects
This is an important one.
Add some way to make objects immutable, preferably at compile time. For instance Rectangle, Point, Dimension NEED immutable versions. Not mainly to improve performance (but it would, since defensive copying would be unnecessary) but also to prevent those subtle and hard to find bugs.
Making immutable versions of the classes mentioned is hard since they expose the fields, but there are ways. I personally like immutable versions of key classes rather that adding a const keyword, but either way is better than now.