Perhaps the biggest thing to set Delta Green apart from Call of Cthulhu as a ruleset is the introduction of Bonds. Instead of some nebulous 'associates' the Keeper might deign to let you call upon and use in some undefined manner*, Delta Green gives you the ability to push all your post-traumatic stress and psychological damage on your friends and family, ensuring you become an increasingly broken and lonely sack of human waste, whose only reason for existing is to protect people you can never relate to, from entities you have begun to understand more intimately than your own hopes and dreams.
Yay.
My only real issue with the Bond system as written is that the rules provide for calling upon Bonds as assets you can pump for material (truth) but don't specifically set them out as living NPCs with their own skillsets. In a campaign I ran a few years ago, instead of going through their 'official' channels, they phoned friendly Bonds to dig up dirt on baddies, got their associates to leave much needed goods at dead drops (no questions asked... or at least, few questions asked), and - in theory - provide overwatch with a sniper rifle at meets. That last one didn't happen, but boy it could've been fun.
So here's my homebrew on making Bonds more useable in Delta Green.
(* that's not an attack on CoC, mind you - I love Call of Cthulhu)
