>> being part of a large backlink scheme
backlinks from various manual blog posts to own site
(linkback in post, not linkback in comment) ?
forum signature link <- i guess not
And i assume if google will just lower ranking for a poor content site or cover less pages in the search index rather than ban complete site from search result.
Regarding quality, it is very possible for certain parts/section of the site is very good and useful and other parts of the site not so good. Depends on where the visitor is landing to the website.