I'm by any standard an absolute rookie here and I realize that gives me 0 standing on how things 'used to be', but as compared to how things
are elsewhere HN is a very large improvement.
That might be reflective of how small the site still is but at the same time it seems that the quality of the discussion is well above most tech oriented subcultures out there.
More users will mean more submissions, eventually the 'new' queue will go by so fast it will be harder to pick out the good stuff, hopefully enough eyeballs on the new submissions will keep the spam and the astroturfing down to a minimum.
And keep in mind that even a user with huge 'karma' can have a bad day, it's an average, not an instantaneous measurement of quality.
There is clear evidence of what you classify as 'whipping the froth' (beautiful metaphor):
For instance there have been many attempts to get the google friend tracking feature listed, at some point I saw three submissions on the 'new' page at the same time and a whole bunch of them that had already passed by, clearly that is not helping at all but it does not seem like anybody cares enough to do something about it. The more that gets a foothold the harder it will be to get rid of it.
Outright spam could be easily handled by making it impossible to submit links until you've crossed some karma threshold, linkfilter.net has used that method fairly sucessfully for years.