Google wants a 1999 web where most just published for fun, out of interest or academically. Links where then a measure of interest in the site. Anything like dealing no dealing, reciproce or broke isn't what they want.
To maintain their link thingy their only aim can be to exclude any commercial linking of any sort. Anything else willbe tarnished with some selfishness. ;)
So I guess i would throw out by default all forums, wikis, blogs, myspaces and so on. Then you remove all .com .whatevernewtldripoffwasinvented sites and leave the .gov .mil. edu sites. possibly leaving old sites valid. Pretty much what they have done.
Think 1999 :)