Thanks for responding folks. I'll answer some issues raised, and raise some more. I've labeled sections to make it easier to read this likely long post. Feel free to ignore those labels.
THE SPECIFIC EXAMPLES
I was just giving some examples off the top of my head, not trying to focus on the details of each so that we could examine why each individual example is a hasty generalization and therefore avoid discussing this issue.
I'll give a better example.
NEW EXAMPLE
A long time ago (many years ago) I threadcrapped in a Buffy the Vampire Slayer thread, in the precursor to the current media forum. The thread topic was something along the lines of "Why we miss Buffy the Vampire Slayer". I went into the thread and talked about how I hated the show, how awful the show concept was, etc. (I don't anymore by the way). I was in the wrong in that case. I had no interest in the topic itself, and was only going into the thread to ruin everyone else's fun of discussing that topic. I deserved to have at least a warning in that case, in my opinion.
That's the sort of thing I am talking about.
WHAT I DO NOT MEAN
I am not talking about ownership of a thread because you start it (and I am sorry I mentioned a thread I happened to start - my intention was not to whine about my own thread, it's just an example that came to mind). I am not talking about natural topic drift either. I am talking about folks who enter a thread with no interest in the topic, but with the apparent purpose of essentially spoiling everyone else's enjoyment of that topic by being contrary for the sake of being contrary.
ANOTHER EXAMPLE WITH CONTEXT
I feel like that sort of thing is happening more often these days. It crops up the most in the rules forum, but it's happening in more than just that forum. Some people seem to go into threads that discuss a topic they actually don't like, just to tell others why they are wrong for liking that topic. If someone brings up a topic about how they like the Book of Nine Swords, and things you can do with that book, it is inevitable that someone who does not like that book will enter the thread and tell everyone how that book sucks. And I just do not see how that is helpful to anyone else in the thread.
WHAT IS NORMALLY ACCEPTABLE IN CONVERSATIONS
I'm sure rules-oriented game players can come up with some plausible excuse as to why that is helpful, but in everyday real world conversation that sort of behavior isn't really acceptable. If a group of people are talking about a sports team and a particular game, and some guy came along and interrupted the conversation just to tell everyone how much all sports sucks and that particular sport sucks, I doubt anyone would consider it appropriate behavior. Nobody would be nodding along that such a contrarian view fostered more in-depth conversation on the merits of organized sports in general. Instead, it would be seen for what it is - some jerk trying to tell everyone else that the things they like are not valid.
THREADCRAPPING CREATES LONG, EXCLUSIONARY FLAME WARS
This sort of thing I am referring to most often takes place in very long threads - the kind that get so long that few people actually read the whole thing all the way through anyway. It often gets reduced to a handful of people responding about some extremely fine details that almost nobody cares about anyway. This can become exclusionary, because of the investment of time it would take to even get to the point where you could contribute to the topic, and because you would be entering a discussion where people's emotions are flaring.
One reason for this is that thread crapping intentionally spawns an angry reaction from the people whose views you are poking at, and people who are upset are more prone to a back and forth angry debate (even if it has nothing to do with the topic at hand). Sometimes, that seems to be the point of thread crapping (pay attention to me instead of your topic).
TWO THREADS IS MORE EFFECTIVE THAN ONE
In most cases, two topics would have resulted in a much better formulation of ideas than one. If a person creates a thread about how best to equip a character with magic items, and you feel magic items are too prevalent in D&D, then two threads would actually result in better discussion on those topics than one: 1) How best to equip a character with magic items, 2) whether magic items are too prevalent in D&D. People interested in each topic would be attracted to the topic because of the thread title. If both topics were intermingled in one thread, half of the people would have no way to know the topic they are interested in is even taking place in that thread, because the thread title has nothing to do with that topic. It would also be less likely to result in a long and exclusionary flame war, where neither topic gets a full discussion because many people are scared off from the discussion (either because of the angry tone, or because of the investment of time to get to the point where they could contribute, or because of the perceived lack of interest from others in discussing the actual topic, or some combination of the above).
WHY THIS IS THE BEST SOLUTION
To sum up: threadcrapping is bad because it intentionally pisses people off who are trying to discuss a topic, reduces topics to flame wars, scares people off from topics, and it isn't the way to get the best conversation going about the two issues. If a group wants to discuss how X is good and what can be done with X, and a second group wants to discuss why X is bad and why we should get rid of X, both groups are best served by two threads, one for each topic, rather than one thread. Each group will know which thread is discussing the topic that interests them. Neither group will get upset because people from the other group interrupt their conversation and call attention to themselves rather than the topic. Threads will not grow as long and exclusionary as often (though it will still happen). People searching for that topic in the future will have an easier time finding the one they are looking for. In many ways, this sort of thing would be helpful.