Java (at least prior to Java 7), has some severe limitations in how code can be abstracted. As an example, it doesn't allow you to pass functions as arguments to other functions directly. As a result, the canonical way to handle situations where you would do this (such as passing a comparison operator to a sort function), is to declare an interface, define the function in a class that implements that interface (command pattern, etc.), and then instantiate an instance of the class and pass that. Closures are imitated by explicitly passing data to class constructors and storing it in member variables, and currying imitated through patterns like the onerous abstract factory pattern.

Additionally, the abstraction features Java does offer are implemented with some heavy language syntax, and often can't be generated at runtime or pre-compile time, at least not easily, and so there's a limit to the abstraction, even taking into account Java Generics. That's something where homoiconic languages like Clojure have a particular advantage.

As a result, abstracting things in these ways (which in other languages is incredibly common) gets to be painfully verbose. Often, things that would be beautifully abstracted in these ways are just left unabstracted (as in-line boilerplate) - the verbosity introduced makes the code less readable, rather than more, and so there's a stigma against "over-abstracted" code. The result is that code that does, or could, make use of these things takes exceedingly more code in Java than in a language like Clojure. Of course, conventions in Java of, e.g., giving each curly brace its own line aren't helping its case in line counts, either.