The quality and intelligence brought to the forums are what we, the users, bring to them. I haven't been on here particularly long, so I'm hardly qualified to address the subject of how much the community has changed, but I personally have found this site to be a remarkably sane and intelligent one for the duration of my time here. True, you get some threads or posters who don't do a great deal to further any deep intellectual debate, or even the sense of fun and enjoyment, but there are a lot of intelligent and open-minded people on these forums, and I love reading many of the discussions that go on. It's not perfect (nothing is, or ever should be), but it's still pretty damn good, in my most humble opinion.
On another note, perhaps one reason that a lot of intelligent discussion is slowing down may relate to the fact that people are always called out for threads that have been made before. I understand this practice, and it is certainly justified, but at the same time, you have to wonder how many times the average person is likely to come up with a completely original discussion point that nobody has yet addressed. I mean, realistically speaking, it's likely that the average person doesn't have as much as a single thought that hasn't already occurred to somebody else in the sands of history (or that the few original ones are either ill-informed or not very discussion-worthy). A repost of a thread that has already happened before does have the benefit of revitalizing a conversation that had wound down simply by virtue of the fact that newer discussions had smothered it, and pushed them out of the view of other users. I'm not really advocating a 'don't check the search bar policy,' since that would be chaos, and indeed cause the site to become little more than repetitive nonsense, rather I'm pointing out one reason why well reasoned and intelligent debates are becoming a rarer beast - simply because there are fewer to have.