Well, first, the core Scheme spec is tiny. Tons of people have written at least half-complete Scheme interpreters (self included, in OCaml). It's a fun project, and it teaches you a lot about programming, even if you don't do the tricky parts (like full continuations). This seems to feed a strong "just fork your own" sentiment in the community, though, and that probably leads directly to a mishmash of incompatible libraries. That's Scheme, though, not Common Lisp; CL was an attempt to reign in several divergent Lisp dialects.
Also, I think people overuse macros. Certain people have put a lot of time into promoting them as the biggest asset of the language, which tends to encourage people to use them where they're not only unnecessary, but likely to make code harder to understand / maintain. (See, for instance, http://news.ycombinator.com/item?id=542746. Especially: "Q: What do you get when programmers design a language while trying to get something else done? A: PHP")
While I'm not knowledgeable enough about Ruby to say for certain, I could see making a big deal about "monkeypatching" causing similar problems there. Maybe the Ruby community encourages more restraint, though. Anybody?
Also, I think people overuse macros. Certain people have put a lot of time into promoting them as the biggest asset of the language, which tends to encourage people to use them where they're not only unnecessary, but likely to make code harder to understand / maintain. (See, for instance, http://news.ycombinator.com/item?id=542746. Especially: "Q: What do you get when programmers design a language while trying to get something else done? A: PHP")
While I'm not knowledgeable enough about Ruby to say for certain, I could see making a big deal about "monkeypatching" causing similar problems there. Maybe the Ruby community encourages more restraint, though. Anybody?