Java is the most used language in the known universe. It is also a very simple* language. Some people believe that there is some scope in enabling Java to accommodate more programming styles (e.g. functional programming via closures). Others believe that being lightweight is a virtue.
The Java language is challenged by languages such as Ruby, Groovy and even Scala, all of which support closures. The majority of people that have spent serious time with these languages prefer them over Java for at least some set of problems.
Features vs. Complexity:
Joel noticed that there is a big difference between good developers and bad developers. Google noticed it with it's picky hiring process. On many projects there are a few developers that do all the real work, while others just slow things down.
But much of the "keep complexity out of the language" debate is aimed at helping the people that write less code anyway.
Nail-guns and angle grinders are dangerous tools, but the building trade accepts the danger in the name of greater productivity. In software development where the difference between good and bad developers is that much greater, and where developers don't actually get physically maimed, the argument for power over simplicity is that much stronger.
In summary, extra power helps smarter developers work more quickly, but makes life harder for less smart developers. Since a small number of smarter developers generally do better work than a large number of no so smart developers, we should tend slightly toward giving developers more powerful tools.
In Java, BGGA closures enable higher-order programming. It's hard to say that the addition of closures in and of itself would cause a complexity implosion, after all, Java is virtually the only modern day programming language without them. It could be argued that the Java language can't be evolved to accommodate them, but I'm leaning towards there being a net gain by including them.
Interestingly, Google Android contains a clean room re-implementation of Java. If Sun (or the JCP) accepts closures, and if (as seems likely) Google votes against them, what will happen to Android? Will it follow how Google votes, or how Java evolves?
*: For some value of simple. The exact definitions of simple used in these 2 paragraphs are not necessarily the same.