Some threads are really interesting to me, but not to many others. I have no interest in others that many don't seem to be able to get enough of. Just because I like something doesn't mean that I expect it to get much of a response. Just because I'm not interested doesn't mean that I expect a thread to fall flat. There are numerous topics that I'll rarely, or never, post in because I have little or no knowledge of them or I'm simply not interested.
There's little point in second guessing which way a thread is going to go. The exact same thread could easily go in a significantly different way depending on the timing of its posting. Who posts it, and who answers, is also going to play a part.
As far as thread deviation is concerned, it happens. Provided it's not a deliberate derailment, for whatever reason, I don't really mind. I consider it fairly unlikely that a thread is going to go exactly the way an OP might have envisaged at the outset.
As for forum cliques, I pretty much guarantee that anyone an individual considers part of a clique won't think that they are. The reality is that if cliques actually exist, they are purely down to the perspective of an individual viewer. Wherever a certain commonality, on a specific topic, is displayed by a group of posters, someone will see that as a clique. The truth is that such cliques, and their make up, shift like sand in a desert depending on the topic.
As for post count chasers, I'm far from convinced that they genuinely exist. Individuals may question someone's motives for posting when, for whatever reason, they dislike a post, but I very much doubt that anyone posts purely to increase their post count. There's nothing to be gained from having a high post count.