I agree with most of the points - documentation is well covered by IDE's, yep, I guess (don't use IDE's myself) ... finding functions is also pretty solved (personally I just use grep).

But I disagree with the part about small functions and the example being contrived because real world functions aren't small and you can't just get your output directly like that.

First of all, functions should be small. Anything you can unit test, you can put in this sort of instant feedback. It's basically just a unit test! Input goes in, output comes out. Without side-effects or other horrors.

Okay, so not all functions make sense to be unit tested. Sometimes you need an integration test and the function can't have direct output like that ... you have integration tests don't you? Why not just evolve the "Instant feedback" feature to being your set of tests?

I honestly don't care about much else in the LightTable demo. But I love the idea of instant feedback. Especially when working with Haskell or something.