[mokus]

As I understood it from the context, the point of the quoted comment was that, even for the _same person_, whether something is noise or not varies wildly depending on their state of mind but for spam that is much less true.

Different people might have different opinions about what is and is not spam, but one person will be fairly consistent about it. For example, it doesn't matter much what mood I'm in - I'll almost always classify the same things as spam. But the same cannot be said of classifying things as "interesting" or "not interesting".

Your idea about a preferential ranking system would probably be a good way to go. Definitely better than trying to train a strictly go/no-go filter to recognize "interesting". The way Google+ posts seem to jump around every time I open the page (with no obvious correlation with things like number of comments or +1's), I actually wouldn't be surprised if they are already doing something like that.