You've been speaking the language and living in Japan (IIRC) much longer than I have, I imagine you have a much better grasp at the language so will defer to you. However, I'm kind of curious about what the contentious points of my pst are. I thought it was fairly well established that Japanese is fairly high-context, and that this can give trouble to speakers who use languages that are really verbose (using pronouns, subject verb agreement, etc). If I gave the impression that Japanese is an utter mess to understand, I was doing so half-heartedly as a joke. In most situations Japanese very easy to understand, and makes me question the necessity of many features of English and French which I speak. Other languages can be very vague as well, much of the time we never completely communicate our desires. For instance a common American English phrase might be "I need to get into the fridge" but obviously does not mean that you are compelled to move your body into a refrigerator, but instead means "move".
I only ask for my own edification.
Actually I'm not an authority on the language by any means, and my level has been deteriorating over the past few years for lack of practice. Other people in this thread like Zefah will provide you with much more educated arguments than I could (and mind you, they might not agree with me). I did used to live in Japan, but 'only' for a year, so I'm hardly the expert. At any rate, I wasn't implying that you're completely off the mark, sorry if that's how I came off.
I asked for examples because Koriandrr was speaking about vocabulary specifically. Off the top of my head I can't think of any words that would be so ambiguous as to be unique (compared to other languages). However, I do feel that people (not you necessarily) tend to exaggerate the vagueness of the structure/syntax/grammar of Japanese. Sure, learners of the Japanese language (including myself) will often scratch their heads, but after a while you realize that the impression of vagueness and ambiguity is there because the language is fundamentally different from, say, English or French (my mother tongue).
For instance, the fact that the subject of a sentence doesn't have to be expressed and that there are no conjugation marks to compensate (unlike Spanish, German, French...) can be seen as confusing from our perspective, but that doesn't mean the system is ambiguous to Japanese people, because they have an intimate, intuitive knowledge of the language. I might be wrong, but I don't think I've ever seen a Japanese person losing track of who is doing what because of this. If they did, it's because the sentences themselves were poorly phrased.
Going back to vocabulary, I saw this comic strip on guidetojapanese.org about the word 彼女 which can mean "she" or "girlfriend". I don't know if this is the kind of words Koriandrr had in mind (which is why I'd like him to give me a few examples
), but as ambiguous as this can be, lo and behold, it's not unique to Japanese. French is the same: when you say "ma copine/mon copain", it usually means "my girlfriend/boyfriend", but it literally means "my friend" and could be interpreted as such given the right context. Every language has its ambiguities.
I guess our disagreement stems from the fact that you're speaking from the point of view of a learner (in which case I agree with you 100%, Japanese is confusing as hell), and I'm trying to speak from the point of view of a Japanese person (which I'm not). From that point of view, I'm inclined to say that the language isn't any more or less vague/ambiguous than any other language, only it requires more work on our part to finally get it. I mean, you have this language where verbs are always at the end of sentences, yet the Japanese don't finish their sentences half the time! How does that make sense? Well, it does, or at least, the Japanese don't have a problem with it, but I'm rambling at this point. I suggest reading Jay Rubin's excellent
Making Sense of Japanese. That book is what helped me in my quest to 'get' the language.