Soleron on 14 April 2009
I don't know of any guidelines, but I would suggest that DLC that is more like expansion packs (changes gameplay or adds a significant new mode) should go in, and DLC that is on the level of mission packs or new weapons should not.
Also, freely downloadable DLC (i.e. that could be called game patches) should be counted in with the original game even if it adds extra modes or gamplay.
