Actually i think that as long as discussing the update or certain features (known/unknown/possible/wishlist), anything should be fair game.
Most of the times discussion devolves into point scoring matches between people because they can't accept a different opinion, not because of the points made.
In that sense, saying the clouds are fantastic is just as fine as saying they look bad to you. This is ok to do.
Saying they suck is just like saying that people who don't like them are blind. This is rude and not ok, regardless to whom it is directed (developers or community members).
Jumping on people's throats for saying either of the two is definitely not ok and it's the biggest issue. Even long time forum regulars get caught out by this, especially when responding to a comment that was delivered in a rude way. For example, if someone says he thinks the clouds need work another guy might chime in to say they look fine to him, but if the 1st guy says they suck then the 2nd guy is all the more likely to get sucked into an argument that turns personal.
So, to keep it brief:
1) Give your opinion in a polite manner.
2) Accept opinions different to your own as being another member's personal taste and not a case of right or wrong, especially if they are not rude.
3) The most important point of all. Don't get offensive with people you disagree with or who are being rude themselves. If you are offended, take it up with a moderator or the other guy via PMs, or state your position in a diplomatic fashion on the main thread. You can put people in their place without cursing and if they do keep being confrontational they end up looking like idiots, not you.
State your opinion but give them the benefit of doubt.
Don't tell a guy he's an internet fascist, tell him he seems to have trouble accepting different opinions.
Don't tell a guy he's blind, tell him that in your personal interpretation that screenshot looks different.
Don't tell a guy he's asking for useless stuff, tell him about the other features you'd be more interested in and aknowledge that all is subject to hardware compromises.
In short, don't assume an air of superiority, don't pretend your opinion somehow values more, don't act like you know it all, don't act like you know what the developers think or consider useful, don't act like the arbiter of what's worthwhile and what's not to post about, don't pretend to represent others (developers or community members) or speak on their behalf and finally, don't pound the other guy's opinion to the ground and get him all defensive and arguing when you can simply state your own view on things
This is how all these circular arguments start. Someone posts something, anything, and before long there is someone else who disagrees but instead of trying to debate the opinion he sets off trying to ridicule the person instead, from which point on it just goes downhill.
I think what's posted in the update threads is 95% within what should be posted. It's usually the delivery and the level of tolerance that are lacking.