There arent that many differences
I own the PHB, CoC, and d20 Modern. The greater bulk of the system is largly unchanged. However, there are some things that I have noticed that would make a great deal of sense for a D&D game.
First, the way Modern handles the Jump skill is a great deal cleaner then how it is handled in the PHB. There is no awkward calculation based on character height in d20 Modern. There is also a Wealth system (which I like a great deal) which can replace keeping track of GP and makes minor purchases like paper, needles, pots and pans a great deal easier to work with (though in modern society, the wealth check makes sense due to banks, credit cards, and other such things, whereas mideval societies are a cash and carry only situation). There are also mechanics for Reputation and "Action Points" that could be brought into D&D for those who care for them.
Cthulhu does not have quite so many elements that would be brought into a D&D campaign. The Sanity system is one aspect of the game that is great fun, and could probably work inside of a Ravenloft campaign. There are also some VERY dangerous monsters in Cthulhu that a D&D player would never recognize, and would make for a great combat suprise. Skill emphasis gives a +3 bonus to a skill instead of +2 like the D&D version.
For my opinion, other then the Wealth system and a few re-worded and cleaned up skills, and maybe a few of the feats, there really is not much that should be added to D&D.
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