Not really anything for sure. You can rule out nofollow, blocked by robots.txt, cloaking and stuff like that. But Google has other ways of devalueing links that we can't always account for. If it shows up as a Google backlink, then it's gold.
This is correct, rel="nofollow" does not pass "link juice" pr and the SE's will not follow that link. The real question is will it create an "orphaned" page(s) (navigation tree). This is the newest link trick I have come accross.
I agree. I've got a number of nofollow links listed in Webmaster Tools, so I'm sure it's not "golden" just because it's in Webmaster Tools.
Well heres what i do, first look through your serps, write down where you rank for what keyword then go buy a link and in a weeks time review your serps again if you have moved up then you are definitely getting some link love.