Obviously, google guys are not liking link building and link exchange. What are your thoughts....how google can stop this. 1. They can consider text to links for external domain ratio. If ratio is low, those external links can be neglected or weightage should be very low. 2. This can kill directories. So its required to differentiate directories and sites. So, If all pages of any site are giving links to external domain in specific number, it can be considered as directory. 3. If a site is giving link to another specific site near the end of all pages with same keyword, all links can be neglected.
Keeping all the mathematics aside - Google should not have any issues with directories which are buliding up database of relevent resources. Directories are supposed to be indexes providing listing of thematically similar sites in relevent categories - and if a directory is following this model there should not be any reason for G to take a negative stance.