I don't think people have any sort of capacity to fully describe their entire audio/video experience in full resolution, but if you think about the real barriers to more limited communication I predict that you'll be able to imagine plausible attempts to circumvent these barriers for the specific purpose of developing a model of a particular real world domain in common with someone with enough precision to derive similar strategic conclusions in limited domains.
I can't define Xs and you can't define Ys, but we know them when we see them.
Maybe I'm misunderstanding you, but my impression is that this is what extensive definitions and rationalist taboo are for: the first to inspire words and the second to trim away confusing connotations that already adhere to the words people have started to use. The procedure for handling the apparently incommensurable "know it when I see it" concepts of each party is thus to coin new words in private for the sake of the conversation, master the common vocabulary, and then communicate while using these new terms and see if the reasonable predictions of the novel common understanding square with observable reality.
A lot of times I expect that each person will turn out to have been somewhat confused, perhaps by committing a kind of fallacy of equivocation due to lumping genuinely distinct things under the same "know it when I see it" concept, which (in the course of the conversation) could be converted to a single word and explored thoroughly enough to detect the confusion, perhaps suggesting the need for more refined sub-concepts that "cut reality at the joints".
When I think of having a conversation with a skilled rationalist, I expect them to be able to deploy these sorts of skills on the most important seeming source of disagreement, rather than having to fall back to "agreeing to disagree". They might still do so if the estimated cost of the time in conversation is lower the the expected benefit of agreement, but they wouldn't be forced to it out of raw incapacity. That is, it wouldn't be a matter of incapacity, but a matter of a pragmatically reasonable lack of interest. In some sense, one or both of us would be too materially, intellectually, or relationally impoverished to be able to afford thinking clearly together on that subject.
However, notice how far the proposal has come from "talking about politics in a web forum". It starts to appear as though it would be a feat of communication for two relatively richly endowed people, in private, to rationally update with each other on a single conceptually tricky and politically contentious point. If that conversational accomplishment seems difficult for many people here, does it seem easier or more likely to work for many people at different levels of skill, to individually spend fewer hours, in public, writing for a wide and heterogeneously knowledgeable audience, who can provide no meaningful feedback, on that same conceptually tricky and politically contentious point?
Summary: I propose we somewhat relax our stance on political speech on Less Wrong.
Related: The mind-killer, Mind-killer
A recent series of posts by a well-meaning troll (example) has caused me to re-examine our "no-politics" norm. I believe there has been some unintentional creep from the original intent of Politics is the Mind-Killer. In that article, Eliezer is arguing that discussions here (actually on Overcoming Bias) should not use examples from politics in discussions that are not about politics, since they distract from the lesson. Note the final paragraph:
So, the original intent was not to ban political speech altogether, but to encourage us to come up with less-charged examples where possible. If the subject you're really talking about is politics, and it relates directly to rationality, then you should be able to post about it without getting downvotes strictly because "politics is the mind-killer".
It could be that this drift is less of a community norm than I perceive, and there are just a few folks (myself included) that have taken the original message too far. If so, consider this a message just to those folks such as myself.
Of course, politics would still be off-topic in the comment threads of most posts. There should probably be a special open thread (or another forum) to which drive-by political activists can be directed, instead of simply saying "We don't talk about politics here".
David_Gerard makes a similar point here (though FWIW, I came up with this title independently).