@Falconne,
I agree that there people just as bad on both sides. I've seen users who feel positively about Civ 5 have a very hostile reaction to players who suggest that Civ 5 is flawed. I feel that there is more than one way to be right. People who like Civ 4 have legitimate reasons for liking it, and people who like Civ 5 have their reasons as well. It's just a difference in values.
I don't mean to suggest that we should assume the general reception of Civ 5 is positive just because of lack of evidence to the alternative. Most of the people I've talked to like a lot, and while that certainly is not statistically significant, it's good enough for me in my personal life.
As for the other assumptions, I don't think anyone suggested that Civ 5 fans don't come to the forums. It was suggested that they come to the forums less often in general, presumably because more of their computer time is likely spent playing Civ 5. It's a reasonable suggestion, but it may or may not be true.
What is far more likely is that the wording of the threads may lead to biased responses. For instance, an actual poll on the forums says "Who else agrees that Civ 5 has been dumb down?" The phrasing of this clearly leads to a biased response, as people who agree will likely click on the thread, and people who don't agree will likely just avoid it altogether. People who try to post a positive comment in that thread will usually be attacked by others who disagree with their view. They will feel alienated and leave the thread, and that's one more opinion that may go "unnoticed."
The thread title has a lot to do with the kind of people that get involved in it. If you open a thread that portrays Civ 5 in a positive light, you will attract more users who feel positive about Civ 5. The same applies for negative threads. People travel in packs =)