This probably isn't the sort of thing that pg is talking about, but what I would like is a large and modern standard library.
I do most of my exploratory programing in python these days, not because I like it better than CL, but because I don't want to spend 6 hours tracking down a library that does X that is compatible with CL implementation Y I happen to be using. It seems like the only truely universal extension to CL since the ANSI spec was written is gray streams. The best case scenario is I find an asdf package, and it happens to work with the implementation I am using.
However, most of the time I'll install it on CLISP and it won't work, then try it on SBCL and it does (or vice-versa).
With Python, I just load up the module reference and spend 5 minutes finding the module that ships as part of the distribution that does what I need.
CL doesn't even have a standard way to connect to a UDP socket. That's a minor thing in and of itself, but I've never tried to prototype any significant program and not run into something like this.
I think the creators of Arc get this, since one of the things included is a webserver. Also, having a central clearinghouse for the language and having the implementation be the documentation both lend themselves well to allowing a large standard library to grow, so I am very hopeful.
After all, Python did not have any sort of impressive library in 1991 when Guido first publicly posted it (it did have a module system though hint-hint).
Seconding on the module system. It may not be the biggest concern of a young language, but it's a big part of the infrastructure that will let the language grow.
And don't forget good documentation, preferably built-in. Whenever I am exploring something I haven't done before, it's a great boon not having to read the sources, or even look it up on the web. Python's help() is wonderful.
