The Blizzard brands also have a massive hold in terms of player attention. Other than the names, I'm not sure it would be exceedingly difficult to re-create CoD style gameplay in another game. I mean even CoD isn't so unique, just critical mass and popularity over time.

In the end, it's the brands themselves that hold customers. They carry from release to release, good and bad. That's a harder ground to make up for than gameplay or style.