n00b1 - 11:19 am on Aug 16, 2011 (gmt 0)
I really don't think there is a general rule (as seems increasingly the case with Google). My theory is that if you have a relatively high number of external nofollow links on a page (or a number of pages) compared to the number of exernal dofollows then that counts against the 'quality score' of the page. If you replicate these kind of pages across the site (not in terms of duplicating content, but by using a similar 'template') then these can really start to count against you. It is kind of like walking into a shop and buying a product that has nothing but an entirely white label on it - you just wouldn't necessarily trust it.
Likewise it could even be that because I was affiliated with a large and trustable "brand" (which I do outrank for my key terms, incidentally) that these outbound followed links to 'authority' were seen as a good thing in themselves.