I have a large software website and on each application we let users add comments, reviews and so on. Each of these pages are called for example "Comments About Firefox", "Firefox Reviews" and so on.
If we don't have any reviews or we for some reason KNOW that the visitor from Google would be disappointed should we add "noindex" to that page? Or should we just let Google decide if they want to rank us or not?
If you know the content is low quality then blocking it isn't a bad idea. That will make you less susceptible to potential Pandaenalties. But only do that if you're sure that content is low quality. Sometimes comments contain useful information that someone may be looking for and by blocking it you're missing out on targetted traffic. Pus Google is good at deciding if a page deserves to rank well or not. Comment pages like yours tend to rank poorly except for long tail searches anyway so having them indexed, or not, isnMt going to make a huge difference for your traffic one way or another.
If you have a review page with no reviews, then I'd definitely consider noindexing it.
Or, better yet, have the script that generates the page return an HTTP 404 Not Found status, since that's what such a page essentially is: a placeholder for a missing actual page.
This is e.g. what Wikipedia does if you follow a link to an article that doesn't exist yet (and you don't have permission to create it — otherwise it gives you an edit form, which won't have a 404 status but is noindexed).
Edit: As it happens, I just came across this relevant recommendation in Google's guidelines on duplicate content:
"Avoid publishing stubs: Users don't like seeing "empty" pages, so avoid placeholders where possible. For example, don't publish pages for which you don't yet have real content. If you do create placeholder pages, use the noindex meta tag to block these pages from being indexed."
