Here's my thinking on it:
"I wish you wouldn't" starts with both parties on the same level, because the speaker is communicating a personal wish of theirs, kind of like "I would like to eat a persimmon".
It is certainly an expression of disapproval, but there is no power imbalance implicit in the wording, and because of that such a statement does not, to me, create a "telling someone what to do" situation. It does put social pressure on the interlocutor, but I don't think it's "heavy" enough -- i.e. it's made clear by the wording that compliance is not required (except if the two people involved in the conversation know they are at differing power levels a priori, and the speaker is in the more powerful position).
So yeah, it can definitely be a "do this or else" kind of statement, but not with words alone; I think it requires very specific appropriate context to be interpreted that way. If I hear "I wish you wouldn't" from someone I don't know (or don't know well), I interpret it literally and believe that what they mean is "I disapprove of your behaviour, but I also recognise and acknowledge that you are free to do as you wish".
OTOH, "you shouldn't" implies that the speaker has a certain amount of authority over the person she is addressing OR (at least) on the subject the two are discussing, because I think "should" in itself implies the existence of a set of rules or guidelines the speaker is not only privy to, but also feels entitled to enforce to some extent*. So the usage of "should" creates a power imbalance, however artificial, and thus puts significant pressure on the person being addressed -- and when it comes from a person of actual authority, it's pretty much an order/demand, not a request.
It's not just an expression of disapproval; it invokes social pressure greater than an individual can exert (because rules and guidelines implicitly invoke the spectre of a social sphere beyond the two participants, and the "should" user designates herself as a representative for that sphere).
So when I encounter "you shouldn't" from someone I don't know, I interpret it as "I not only disapprove of your behaviour, but I also feel it is within some of my power to make you correct it."
That's what I think, and I should (ha!) add that because English is not my native language, I tend to interpret things said to me precisely (i.e. literally) within a given context and I tend to try and use precise wording/terminology when I speak (it cuts down on "that's not what I meant" type situations) -- I don't always succeed, but. XD
* Not to the same extent as the user of "must" or "have to" does. I HATE it when people say "you have to" to me unless they're obviously being ironic or funny. I realise this kind of usage doesn't necessarily stem from *feeling* personally superior, but I parse it as hostile and arrogant even if it's coming from a friend. I think "you have to" very strongly implies that you have power over your interlocutor, far more so than "you should" (which I have fewer issues with and use quite a bit, but more often in a facetious context, like "you should totally cosplay Bella Swan!".) Don't get me started on "you need to". DON'T TELL ME WHAT I FUCKING NEED. >.> [/issues]
What do you think! :D
A RT of this: What makes us men is that we can think logically. What makes us human is that we sometimes choose not to. (Roger Ebert). WHAT? First, I am not a man, and I can still think logically quite well, thanks. Second (and most important), "sometimes not thinking logically" is a defining feature of the human condition? I have agreed with a lot of what Ebert has had to say lately, but in this case, I gotta say that affluent white males should be banned from attempting to coin timeless adages, even on the internet. Plus Google tells me that's a phrase from a review he wrote for Searching for Bobby Fisher in 1993. So he's RTing his own spew from over a decade ago? Classy. *headdesk*
The subject line of my post translates approximately to "a body immersed all the way to the bald spot in shit should remain still without burbling, and ask for no money".
...actually that kind of sounds like a Gintama chapter title. \o/ [It's a song lyric that makes sense in context >.>.]