Namespaces for me.  The kind where I can write a new package without worrying
about prefixing anything, declare a public interface and later import the
package with an appropriate namespace prefix.  No more unnecessary namespace
pollution or pondering about the tradeoff between ergonomics and potential
namespace clashes.  Experimentation gets easier, say you'd want to have list
processing functions à la <insert favorite Lisp>.  You can now design such a
thing without forcing a clunky or misleading namespace prefix on everyone.  Or
imagine you'd want to replace or prototype a built-in facility.  Or you'd like
to use just one thing from a package.  All these things are now possible, at
the cost of `grep` becoming a less powerful tool.

Vasilij