Andy_Langton - 6:06 pm on May 4, 2012 (gmt 0)
I imagine the type of navigation considered webspam is likely to be those with many repetitions of keywords.
Information hierarchy, and applying that to website navigation is complex, but I've always found a good structure to pay dividends. The planning stage is where a lot of navigation menus fall down. In terms of usability, the user needs to know "where am I?" and "what can I do next?". Not "what is every possible choice I might make?".
In terms of drilling down, I think it depends what sort of page you have in terms of usability concerns. But getting every combination of filters indexed as unique items of content has never been a good idea for SEO, so perhaps some of the sites you're seeing have finally realised that.