I couldn't agree more. Lots of people are panicking about Unity because it's not as configurable as Gnome use to be, but they forget that we're in a transition period. Unity is brand spanking new, and I'm sure it'll have more configuration options in the future.
That might be a good point if it weren't already so simple to build user interfaces that are simultaneously obvious to newbies while at the same time being ridiculously extensible. You just build it in a language that exposes a repl. Mozilla doesn't even do a particularly good job at this (given how they bury the replness), and it's still fostered a plugin ecosystem that has kept it competitive despite competitors eating its lunch on performance grounds.
Having a REPL and being extensible seem like totally orthogonal concepts to me. You can have a REPL and be hardly extensible. You can lack a REPL and be extremely extensible. If anything, the REPL is going to expose interfaces that enable extensibility; it's those APIs that are important, not the existance of an REPL.
Technically they are orthogonal, but in practice if you have a repl implementation that's well done, it will be used by the developers, and the developers will ensure that the functionality they need to implement the project is accessible through it. The developers can still screw things up, but it's a factor that encourages them to do the right thing through basic dogfooding.
Obviously it's implied that the repl be implemented in the same language as the rest of the codebase. So adding one to a project written in C++ would be a waste of time.
