Great topic, Backdraft7. Crowding is making a mess of the SERPs, and it does seem like spammy stuff and crowdsourced stuff is at the top even when it's not relevant.
Something I'm seeing a lot of lately is irrelevant authority sites. I'll query something, and get results from Amazon and other top domains that don't come close to addressing my query. They have a word here or there that kind of sounds like my query, but the much more relevant results - which may come from small blogs or websites - are buried below the first fold or even on pages 2-3. Sometimes I just start on page 3, which is a trick I picked up around 2005 when things were this bad.
This makes me think that netmeg's right about authority being huge. I think if Google has you down as an authority, they put that WAY ahead of other ranking factors - even relevance, which ought to be the top ranking factor IMO. So maybe the filters you're talking about ARE working, but authority ranking is allowed to overcome them in a big way. That would be my guess.