The problem that I have with lisp macros is that the elegance you gain at the syntax level is effectively a tradeoff with pragmatism when other people read and use the code. Java was developed with parts of C++/C as inspiration and parts of that language were left out. Particularly, operator overloading was left out (which can be viewed as a very restricted example of modifying the language), presumably because it's not an immediate thought when viewing the code that the operator isn't doing what you expect. While it's undeniable that macros make the syntax nice to look at, the same argument that lisp becomes a new language as you write your program means that every separate codebase has a lot more reading to understand because you have to go through all the macros. New releases in most languages introduce new features (and standardize functions, fix bugs, etc), as far as I can see, new releases in lisp enforce a standard (common) base set to decrease the amount of work required in learning new codebases.
Yes, macros can make code very hard to read, if designed badly. Of course, so can functions, variable names, and program flow.
But Lisp with macros is very different from C++ with operator overloading. With C++ operator overloading, you only know if a given line has something you don't understand (that is, an overloaded operator) by looking at every other file in your project. With Lisp macros, you know that you're dealing with something new because you don't recognize the first token in the s-expression. You might not know it's a _macro_ rather than just a _function_, but you know it's something you need to investigate.
Basically, in Rumsfeld's terminology, an overloaded operator is an unknown unknown, but a Lisp macro is a known unknown. A macro's behavior may be confusing, but its existence isn't. And that's a very big difference.
