This may sound silly, but I think what a lot of people construe as a difference in fun is actually a difference in the effort one must put forth to play the game.
Allow me to recall one particularly long session of play on Sicily while I was still at school. I was playing, as was my roommate. Early in the year, both of us had played A LOT of CS. He still did at this point, while I had almost completely switched over to Inf for my online killing needs. Early on in the match, he was cannon fodder, while I, already experienced, was playing very well. As he died early on a few times, he observed the way I played, and he was actually doing pretty well toward the end of the match. The match was best of fifteen, it lasted about an hour and ten minutes, and our team won eight to seven. At the conclusion of the map, we decided to quit, and to our surprise, we were both actually tired! Infiltration had worn us out. It takes more effort to play Infiltration, you have to plan your strategies every round, and your adrenaline level is always high because you fear for your fragile Inf soldier life. I believe the exact words my roommate used were, "It's so much more intense than Counter-Strike." The high level of intensity and greater mental strain is seen as some people as "work," and for most people "work" does not equal "fun", and thus they say, it's not as fun.
Just my two cents on the "fun" issue.