Pure programming joy.
Make sure you read Joel's to the end and then follow his reference to the Kindom of Nouns - well worth the click it will cost you to get there.
So, I really don't need to explain these things ... they did fine, but I want to add my two cents worth ...
First penny: if you're looking for a job implementing programming language runtimes and you only know Java... umm... I suggest you learn a couple more languages starting with C/C++ (so you know how programming carelessly can really hurt hurt hurt you) and then Scheme (so you learn about recursion, first class functions, continuations, and programs as data) and then finally Smalltalk (so you can experience true object oriented programming and pure programming joy).
The downside is you may be spoiled for lesser programming environments, and become surly and depressed once you understand the sad truth of the programming language landscape today. It's not all doom and gloom however, some people are doing something about it like this guy.
Second penny: Steve, Joel... yes, better employment opportunity, better programmers come from knowing other languages but you missed one thing, fun ... we could also have pure programming joy too.