Did those 7+ million guys play up to 20 hours of multiplayer because the game is 60fps?
I think the success of the Call of Duty franchise is difficult to explain, and has many wondering, including many other developers with respective shooters of their own. I don't think it's entirely framerate either - perhaps it's more the point that CoD aims at the market interested in "realistic" shooters - realistic meaning,
real weapons, conflicts based on past/present/future events which perhaps is a bit more attractive than the science-fiction space setting in the KillZone game. The graphics in the CoD franchise is less stylised, more realistic in appeal, which perhaps is another factor.
Perhaps the success of CoD can also be attributed that Modern Warfware was the block-buster game that was simply better than most shooters at the time - compared to Resistance (and some others that were out at the time). The success just carried over and success spreads the word. I think the framerate is certainly a factor, but sadly, not one, that's high on most peoples list. I'm sure that if Activision was to suddenly downgrade their game to 30fps, most people would still buy it - although perhaps many of its core-gamers (the 8 million that were mentioned further up) would probably question "what is off about the game" - not feeling as smooth and responsive as the 60fps CoD before it.
I guess you set the bar with the first game of the franchise (or the first one that really sells). Once you have your franchise known for that responsive gameplay, it's hard to cut down on that feature because people come with expectations.
It's saddening that framerate is not on most peoples mind. I'm sure most people are aware of differences, but tend to lack the technical understanding on "why" the game feels different.
It's a pitty this KZ aims for 30fps. I actually think I would prefer to have it at 720p@60 than 1080p@30.
Actually, I think if you would conduct blind tests on running the two games side by side, one in 1080p@30 and the same game 720p@60, I'd be surprised if people chose the higher resolution one over the one with the quicker framerate. IMO the 60fps would be much more noticable than the resolution increase on the other. But of course, going into next generation, 1080p is of course the big catch-phrase and bullet-point feature. And now that tablets and phones (especially iPhones that started with the retina catch-phrase) are making people aware of ever increasing screen resolutions, it's the new feature everyone wants to push.