I find that Fish is better for interactive use. It has as-you-type syntax highlighting and indentation, and the tab completion tends to be better.

I would like to see the interactive Read Eval Print Loops of one of these languages finish closing the gap and become good shell replacements. I find I can get close to that sometimes with interactive programming environments, but not quite.

There's rush: "rush is a replacement for the unix shell (bash, zsh, etc) which uses pure Ruby syntax" from http://rush.heroku.com

I haven't used it though, I actually switched from bash to fish about 6 months ago

Have you tried ipython? http://ipython.scipy.org/moin/ Pretty close...

I've been thinking about this also and was planning this week to change one of my login shells to just pythn to trial it. I've now got two small line-based editors running in python (one is an ed clone, the other is simpler and uses list comprehension), and it's simple enough to write wrapper functions around the bits you need to change directory and spawn process. There's no standardisation of effort, but it's completely doable just on your own system. I realise it's probably not at the top of your mind, but can you think what you found missing when you last tried it?

Well, there's scsh (http://www.scsh.net/), but it's not under active development any more.

Which is a shame.

I just found shelisp [1] which, despite the questionable name, looks simple and promising.

[1] http://dan.corlan.net/shelisp/