Sorry to report this, but you cannot change metadata for the clip, unless it's in List mode. Forgive me if I'm wrong, but I know of no way to make comments, mark good, etc. in list mode for other popular NLEs, like Avid Media Composer, or Final Cut Pro 7 either.
You can make a feature request, if you like: http://www.adobe.com/go/wish
Already requested. Just because others don't do it doesn't mean it wouldn't be *really* useful! It's a fairly simple change.
Good point. One question: how would you mark it good? Context menu? I'm wondering how you are picturing how it would work.
There are already other small icons over laying the thumbnail (e.g. Audio), so a another small icon could be used (as an example). There is also lots of room underneath (where you can rename) to have a check box or some other simple indicator (to the left of the clip length). You could even change the clip length text colour if you were short of space.
Allowing Alt-G (or other keyboard short cut) could toggle it on/off if you don't want a check box.
'Good' could also be (say) the top item in the popup menu (right click) (please don't bury it lower down!) though I realise that's probably a generic menu that's called from lots of places so it's more work to make that happen.
Lots of ways to do it. Let the programmers choose the path of least resistance
Good ideas, and thanks for the feature request.