CoD4 was not the first popular CoD game. That defintely was the work of CoD2. Now to add onto that, the first 2 big CoD titles were critically and PC-user acclaimed for being fantastic for their time. CoD4 was the first massive commercial success, and that was because the original IW Staff fought tooth and nail to have creative control over it, realizing people were getting sick of WWII shooters, and wanted something different. You could even go so far as to say without BF2, CoD4 probably wouldn't have been the game it was.
I also refuse to believe that the popularity\sales of a game instantly translates into the quality of the game. I played MW2 for a bit on PS3, and it was only entertaining for about a month or so. Compare that to BFBC2 on the PC, and I still pick that game up and play it now and then. I played Black Ops at a friends house and hated it because it was basically MW2, just reskinned with crappier mechanics.
Don't even get me started on "Hit Registration" in recent CoD Titles, because that is basically a paradox. The reason it's so much easier in MW2 to land shots is because the guns have no recoil, and the guns do not react to external actions. Take Counter-Strike for example, if you run and shoot, good luck lol. In MW2, if you do the same thing, you're basically the worlds greatest marksman. CoD4 wasn't nearly as bad (I played it both on PC and PS3) but really started the trend. The entire game is basically a close-range on rails shooter, where you just run and gun as fast as possible. In BC2, the guns act like actual guns, not the Rail Guns that exist in the CoD-verse, so you are required to apply significantly more skill and intellect to your play.
I have paid pretty close attention to the last several CoD launches, and to say they are a better experience out of the box is also nothing short of laughable. Remember the massive problems with MW2? Some of which are still present in the game, 2 years later? And those same EXACT issues were present at the Black Ops launch. Let's not forget the several glitches that have gone unaddressed for months at a time. On the PS3, the Javelin Glitch was around for about 6 months after it was exposed. I consider stuff like that unnacceptable. When I first popped in my copy of MW2, I had to patch for 10 minutes, got in game, the servers crashed, waited half an hour, patched AGAIN for 10 minutes, and by the time even got into a match with some friends, it was unbearably laggy for everyone.
I'm sure there are plenty of people that have had great experiences with recent CoD games, but when I compare them to games like UT2K4, Counter-Strike, Quake 3, and now games like TF2, the experience just isn't there.