I agree the property reference tab autocomplete is a pretty powerful feature. It's like autocomplete in a shell, but contextual, since the system already knows what you want the first argument to be. And I've seen a video with one of the big functional guys (maybe Brian Beckman?) pining for that feature.

I'd point out, though, that is still is only part of the picture. You're holding on to a value and you want to know what you can do with it:

    widget.<tab>

will show you everything of the form

    widget.someData
    widget.doSomething()

but you're still missing out on other structures like

    freeFunction(widget)
    handler.handleWidget(widget)
    hammer + widget
    widget[part]

    // returns a widget, does this one count?
    widgetFactory.buildWidget()

In nearly any language the first two will be common, and where available the others are critical usages, too. I want to be able to tab complete them!

It's hard to continue hewing to the tab as activation with these other structures, which may be why IDEs and REPLs don't really try.