I assumed this was a reference to the unequal handling of "primitive" types vs Object types, with only the latter being able to be used with generics. Except for the weird "dual" object types which sortof are and sortof are not equivalents to the primitive types (int -> Integer, etc). WHich for methods needing to take actual primitives leads to having to do abominations of repetition such as http://www.docjar.com/docs/api/java/util/Arrays.html.
Some other bad things (not necessarily "complicated" but it does complicate the code that needs to be written):
Enumerations all have to have the same constructor params and therefore the same "shape". SOrt of defeats the purpose of enumerating ALTERNATIVES, I would say (this is part of the lack of sum types that was mentioned, though).
Others mentioned nullability. Some other things I didn't see others mention:
Botched covariance of arrays which leads to runtime failure where compile time should have been sufficient, http://k2java.blogspot.com/2011/07/parametrized-types-and-ar....
Some other minor things, such as botched clonability system, where Cloneable is a "marker" interface that doesn't tell you whether some type is really cloneable, a bizarre unforced error where a simple interface (you know, with an actual method called "clone") would have sufficed.