I think I've got the concept from what I've read so far of Paul Graham's "On Lisp".
Basically what lisp allows you to do is modify the language to suit your program, while building your program around that new framework. So for example, imagine you need constructs specifically for numeric processing - you can build them into the core language's syntax. This is like writing functions, but on steroids.
This sounds ideal for hacker types, and for proficient lispers. The problem with it is the same as if I came to work on some code someone had written using lots of new Perl syntax (say constructed with source filters). If I had to work on some perl 5 code that contained completely new constructs, I think it would be an awfully steep learning curve (even if it was really quick to develop initially for the developers).
Of course it's very different to perl and source filters, since in lisp, everything looks the same (braces with params). So it's not like you can invent a completely new syntax (or I haven't gotten that far into the book yet).
Still, when all's said and done, I think the syntax has been one of lisp's biggest downfalls. As kids we found basic easy to grasp (ok, we found logo easy to grasp - but we couldn't really build anything serious with it
Anyway, just some random thoughts on my recent readings. Hope you enjoyed them