I disagree. I think it's just that threads that acquire negative overtones become confrontational and thus grow into much larger, more notable threads than positive ones where everyone's agreeing with each other. Positive threads rarely tend to last long because there's only so much to say beyond "X is great", "yeah, I love that part of X", "the singing in X is cool". But if there is a clash of opinions, where contrasting viewpoints are trying to present themselves as the most valid analysis, then you have the basis for a much longer discussion. It isn't even necessarily complaining and I'm tired of how some people here are uncritically labelling all negative posts as 'complaints'.
Well, I do mainly listen to other stuff nowadays, and I happen to like the majority of what U2 has released. But this is a discussion forum, and so I will discuss both what I like and what I dislike. I don't see the problem there. The problem comes when someone trolls and just bashes U2 - and those people get banned pretty fast. We're all U2 fans here, at the end of the day, but we all have our own individual preferences within U2's catalogue.