Readability and maintainability are pretty much the only reasons why I wouldn't recommend Lisp for everything. Other than that, Common Lisp has had practically all features you could desire for decades already.
It's not a problem with the syntax, though. The problem is that Lisp is so powerful that programmers can and will create very high level abstractions, the language and conventions encourage it, but a side effect of this is that every part of a program has its own DSL. If in addition there is not good enough documentation or a programmer has a knack for obscure elegance over KISS, then this becomes problematic.
In a nutshell, it's perfectly possible to write clean and easy to read Lisp but not many people do it.
Edit: I forgot to say that the code from Mezzano looks like a positive example of readable and clear-cut Lisp code.
I completely agree, and it’s also a problem that other immensely powerful languages like Ruby and C++ have but Lisp has it far worse since A) the more arcane abstraction features are more immediately accessible from day one of writing Lisp code and B) the culture of Lisp code is far more obsessed with elegance and with twee little tricks than with readability or simplicity. It’s not a necessary issue that Lisp dialects have though: Common Lisp seems to be better as far as culture goes, and Racket has both a more simplicity-focussed code culture and the tools to make the arcane and elegant abstractions easier to understand.
It's not a problem with the syntax, though. The problem is that Lisp is so powerful that programmers can and will create very high level abstractions, the language and conventions encourage it, but a side effect of this is that every part of a program has its own DSL. If in addition there is not good enough documentation or a programmer has a knack for obscure elegance over KISS, then this becomes problematic.
In a nutshell, it's perfectly possible to write clean and easy to read Lisp but not many people do it.
Edit: I forgot to say that the code from Mezzano looks like a positive example of readable and clear-cut Lisp code.