I would somewhat tentatively say that the sentence on the sign is grammatical. My own intuitions don't seem to give an entirely clear answer, but other people's intuitions and usage seem to support this kind of sentence.
I think the objection to the sentence "Queueing is so thoughtful of you" is based on an objection to the use of the structure "[verb]ing is (so) [adjective] of [someone]" to express the idea "It is (so) [adjective] of [someone] to [verb]". The structure starting with a gerund does seem to be grammatical according to the judgements of at least one linguist, but it sounded a little strange to me when I first thought about the sentence in your question, so it may not feel entirely natural for all English speakers. ("Queueing is so thoughtful" sounds OK to me. So it seems to me that the issue, if there is one, would be with the use of the prepositional phrase "of you" after "thoughtful" in this context.)
I found the following apparently parallel examples marked as grammatical in Adjectives and adjective complement constructions in English, a dissertation by Arlene Berman (1973):
(28) a. Burning the papers was clever of you
b. Bringing wine was thoughtful of Mary
c. Continuing to harp on the subject was wrong of you
"Wise-type adjectives" (like "wise of you")
I was led to Berman's dissertation because it was referenced in a paper specifically about the class of adjectives that behave like this: "Wise-type Adjective Constructions and the Uniformity Condition", by Shin Oshima. I wasn't able to find any examples in Oshima's paper that seemed directly relevant to your question, but it has some useful terminology and examples of other constructions that make use of adjectives like this.
Berman's judgements of related constructions
I think to understand Berman's examples better, it will help to look at some of the other things she says about this type of adjective. Actually, Berman's section on adjectives with of-NP complements starts out with examples of extraposed sentences of the type that I feel entirely comfortable with (and that I assume any English speaker would feel entirely comfortable with):
(24) a. It was clever of you to burn the papers
b. It was thoughtful of Mary to bring wine
c. It was wrong of you to continue harping on the subject
Berman points out that sentences like "*It was clever of Max for his sister to do that" and "*It was clever of Max for him to do that" are not grammatical (p. 229). (However, note that Oshima says the latter construction may be possible in some dialects.)
Berman also judges the following sentences unacceptable::
(27) a. *To burn the papers was clever of you
b. *To bring wine was thoughtful of Mary
c. *To keep harping on the subject was wrong of you
(28 is the acceptability judgements given above for sentences starting with a gerund)
(29) a. *It was clever of you burning the papers
b. *It was thoughtful of Mary bringing wine
c. *It was wrong of you continuing to harp on the subject
and suggests that "It seems reasonable to assume that sentences like (28) are derived from sentences like (27), when Extraposition fails to apply.5" (p. 230).
Endnote 5 says:
This sort of alternation seems to exist more generally. I find, for example, the following:
(i) It was easy for Joe to do that
(ii) *To do that was easy for Joe
(iii) Doing that was easy for Joe
Bare infinitives in subject position seem to be quite restricted, although they are not wholly impossible. I have not examined the conditions under which they are and are not acceptable.
I agree with Berman that bare infinitives are not wholly impossible as subjects. I'm not sure that I agree with her judgement that "To do that was easy for Joe" is unacceptable.
Berman also lists a few restrictions that seem to her to apply to this kind of adjective. The first of the following restrictions seems more obviously correct to me than the second. Berman notes that
While other subject-embedding adjectives allow the noun phrases in their prepositional complement to be questioned and relativized, this is not possible with sentences like (24):
(37) a. To whom is it clear that Max is wrong
b. the man to whom it is clear that Max was wrong
(38) a. For whom is it easy to write a dissertation
b. the man for whom it is easy to write a dissertation
(39) a. *Of whom was it stupid to do that
b. *the man of whom it was stupid to do that
Berman has no explanation for these acceptability judgements, but I fully agree with them.
The next judgement seems more iffy to me: Berman feels that
adjectives with of-NP complements do not allow Tough-movement:
(40) a. *That document was careless of Jeb to burn
b. *That was stupid of you to do
I don't feel certain that these sentences are grammatical for me, but "That was stupid of you to do" doesn't immediately seem unacceptable to me either. I found an example of it on the Internet (I don't know if it was written by a native English speaker):
The pisces men I know/knew (March 13-18th babies) are all fragile ego, sensitive as shit dudes. Don't tell him he's not behaving like a man, his brows look bushy in a picture, that was stupid of you to do because then his whole ego is thrown off; he doesn't wanna play anymore and he's taking his ball home with him.
(comment by Peachyvision, Apr 30, 2015; #18 in the thread "So do pisces men like to get cussed out and called names?", discussion in 'The Lipstick Alley Psychic Hotline' started by StayMadBishes, Apr 30, 2015)
Contexts where it sounds OK to me to use a "wise-type adjective" without a following to-infinitive
For me, "[adjective] of you" definitely works without a following to-infinitive when the subject is one of the following pronouns:
"That wasn't very nice of you."
"This is so thoughtful of you!"
"He helped me with my errands, which was thoughtful of him."
(It also sounds fine to me to use a quantifier before one of these pronouns, as in "All of this is very thoughtful of you.")
And it also sounds fine in exclamatory sentences with "How" at the start (I don't know enough syntax to know how these work):
(In some cases, at least when there is a to-infinitive, the "how" can be left out in similar sentences: "How stupid of them to leave it behind!" "Silly of me to forget about that.")
(Also, I don't know if these could be taken to be short versions of "How silly of me that was" or "How silly (?it was) of me to do that")
Other examples I have found of wise-type adjectives used without a to-infinitive (taken from the Internet)
I was able to find "worrying was silly of me" used in a blog post by someone who appears to be a native English speaker: "Internship Assignment A", Keep Calm and Study Abroad.
"Everything you say about my time with Wade is thoughtful of you" (This was the only example of "is thoughtful of you" that I found on Google Books that didn't have "it", "this" or "that" as the subject)
From Google as a whole (not Google Books) there are few more examples with a gerund:
and from Google, a few with other nouns as the subject:
With a plural verb:
John Lawler's answer to How does the to infinitive work with adjectives like “wrong” and “wise”? provides the example
For me to go home that day was wise (of me).
where the infinitive precedes rather than follows, but I don't know if that is intended to be a grammatical example sentence, or just an ungrammatical illustration of the theorized underlying structure of "It was wise of me to go home that day." (Another example is "It was wise (of me) (for me) to go home that day", which as I mentioned above is ungrammatical for most speakers when both "of me" and "for me" are included.)
Lists of "wise-type" adjectives
Berman gives a short list of adjectives that take of-NP complements: clever, stupid, idiotic, careless, wrong, brilliant, kind, nice, thoughtful (from (23), on p. 229).
Oshima mentions most of these, as well as a few more like foolish, good, wise (from (1), on p. 12).
I found another source that mentions "wise-type" adjectives, the SAL Taxonomy for English Adjectives, but none of the related texts that I found mention the "V-ing is [adjective] of [N]" structure specifically:
SAL Taxonomy for English Adjectives (PDF)
SAL Adjective Narratives
This resource lists a number of other adjectives that act like "wise" and "thoughtful": audacious, astute, bestial, bold, brilliant, careless, childish, discriminatory, early, far-sighted, heroic, humane, impudent, intelligent, late, negligent, noble, perceptive, polite, provocative, reckless, rude, shrewd, undiplomatic, unprincipled
Berman, Arlene (1973). "Adjectives and Adjective Complement Con- structions in English.” Ph.D. dissertation, Harvard.
Oshima, Shin (1987). Wise-type Adjective Constructions and the Uniformity Condition." Kochi University Academic Research Report 36, Humanities