The basic idea of the world file is that you have the packages that you know you need there, then by setting apropriate USE flags you let emerge take care of dependencies of those packages. Actually those entries should be rather called "packages' dependency specifications". Anyways try running:
And see what emerge suggests you to nuke. Then you go through that list, add entries, you want to keep, to your world file, check the results by running the command again, and rerun without '--pretend' (remember to revdep-rebuild just in case).
Also 'udept' tool could be of some interest:
(To clean up world file 'dep -w' could be used, but take extra care)