Sasha: Welcome to EL&U (and I mean that sincerely).
When I was in school, there was a common refrain: there's no such thing as a stupid question. The sentiment was, if you are confused about something, you shouldn't be afraid to ask.
I still believe that's true – there's no such thing as a stupid question. However, just about any question can be poorly presented.
I've heard others mention that the "downvote phenomenon" seems more common at EL&U than at some of the other Stack Exchanges. Maybe that's because the active members of other exchanges are more polite, but it could also be because many of those forums get far fewer poorly-presented questions. For example, the Mathematica Exchange has had a little over 1,700 questions asked; Computational Sciences just over 600. The English Exchange is nearing 16,000.
We do downvote a lot of questions, but whether or not such downvotes are a symptom of a rude and unwelcoming community is up for interpretation. There are two issues that need to be investigated: (1) what does a downvote represent? and (2) what kinds of questions are being downvoted?
I find that most downvoted questions on EL&U seem hastily written, and the O.P.'s haven't done a very good job of "marketing" their question. Usually, these questions are either very basic (causing people to scratch their heads, and wonder, "Doesn't this person know how to use a dictionary [or thesaurus, or search engine]?") or else very confusing (because there is insufficient context to adequately explain the question).
Yet downvoting for these reasons is not limited to EL&U. What if I went to Math.SE and asked:
Confused about algebraic operations
What's the difference between addition and subtraction?
How "welcoming" would that community be?
Some might think of a creative response:
Now that you mention it, there's really no "difference" between the two; they are essentially the same operation. After all, 2 + 3 is the same as 2 - (-3)...
but I suspect more than one user would be a bit annoyed at such a basic question, and perhaps one might even be brave enough to openly chide me for asking it. (I don't frequent the Math Exchange, but such downvoting does seem to happen there when users ask inappropriate questions, as evidenced by this example and this example. Moreover, and it's not difficult to find examples in other exchanges as well – observe how this user was downvoted and rebuked, apparently for failing to provide sufficient information; notice the upvotes on the chiding remark as well). Here's another example of an unwelcoming reaction to an off-topic question on another exchange.
All that said, the EL&U community is very welcoming toward questions – even very basic questions – that do a good job of explaining why the question is being asked. Let me point to two examples: a downvoted question, and this upvoted question, both asked by community newcomers on the same day. The upvoted question is actually the more basic of the two, but the O.P. does a good job of explaining why he is asking the question: a concise yet clear context defines the scope of the question, and initial assumptions provide a baseline to work from. If you visit the question, you will see, not only did it reach 20 upvotes, but a few of the answers did as well. In contrast, the downvoted question, which has since been deleted, seemed hastily written, and too open-ended:
Is this the correct way to put this sentence?
"So I know you like coffee, and I know where to find the best coffee
close to your work"
Thanks in advance.
What did the O.P. mean by "is this correct?" In what context would this sentence be used? A formal letter? An email? A conversation? How is anyone supposed to know? After one hour, the O.P. added this comment: "Its [sic] a conversation, asking out for coffee" – that's an important piece of information!
I've seen this pattern repeated rather often: a question is fired off, regulars ask some pointed questions, and only then is the vital information provided as an afterthought. In such situations, I've often thought to myself, "Had that information been included in the original question, instead of posted in a later comment, maybe this would've gotten upvotes instead of downvotes." Unfortunately, some O.P.'s seem unable or unwilling to accept such feedback as helpful; rather than thinking introspectively, trying to figure out how their question might have been improved, they become insolent or confrontational instead.
Going back to the question you cite, when I read:
Is ending an email with a question like that permissible?
my first thought is, "Why on earth wouldn't it be?" (After all, isn't that what the O.P. just did in the question?) But when I see this addendum:
In my opinion unsolicited marketing emails shouldn't be asking any questions at all. They should politely present the information. Question at the end of the email prompts for action, and since this is a marketing email, I will find it an unnecessary distraction. I asked the question here because I have never seen such a thing in Russian emails, where I would consider it rude.
then I think to myself, "Hmmm... that's quite thought-provoking" (it sets a better context, and I also find the cross-cultural reference rather interesting).
All this leads me to my closing point: is a downvote necessarily rude?
Quite frankly, I'm a bit surprised how often an unexplained downvote is considered a breach of etiquette (as opposed to being a mechanism to let O.P.s know that perhaps they could have done a better job framing or researching their question). More often than not, the first downvote isn't meant to be "rude;" it's nothing more than a quick way to express this sentiment: "I don't really like this question, but, before I fly off the handle, let's see what others think."
If an O.P. can accept a downvote or comment as helpful feedback, meant to indicate that a question has inadequate information, or is simply not a good fit for the EL&U forum (neither of which makes it a bad question, by the way), then I think you'll find this to be a very friendly, welcoming, helpful community.