Would you also object to "key witness" and "key evidence"? - this article is related but not exactly about the same thing - http://public.wsu.edu/~brians/errors/key.html

OK - the rise in its use as an adjective is pretty recent in both American English and British English, but its occurrence seems to be more common in BrE.

http://books.google.com/ngrams/graph?content=key+issue%2Ckey+decision%2Ckey+role%2Ckey+element%2Ckey+factor&year_start=1800&year_end=2000&corpus=18&smoothing=3&share=&direct_url=t1%3B%2Ckey%20issue%3B%2Cc0%3B.t1%3B%2Ckey%20decision%3B%2Cc0%3B.t1%3B%2Ckey%20role%3B%2Cc0%3B.t1%3B%2Ckey%20element%3B%2Cc0%3B.t1%3B%2Ckey%20factor%3B%2Cc0

@Skeeter Lewis - I don't really see why there should be any connection between the two - I doubt that many Brits even knew of the American idiom. Far more likely is that they've decided to avoid "less than half-price", just in case.

In fact, it's not as exclusively British as I had thought, as a quick visit to Google Books shows. This is from 1999 - "Alan and Patricia Wolff of Honolulu took advantage of these peculiar economics and snared a better-than-half-price deal on a ski-season time share in Park City, Utah"

1. I vaguely remember reading somewhere that gift as a verb is not uncommon in Scottish English. It is relatively formal, and is especially when something is given officially. This use in Scotland goes back to at least 1602. What is strange is that several publications carry exactly the same sentence, so its provenance is quite difficult to work out, but given the context I imagine it started off in a Scottish publication.

2. Key venue - I really don't see the problem here - a key venue is a very important venue . There will be at least fifteen venues at the Glasgow Commonwealth Games and presumably the writer considers this to be one of the more important ones. Oxford Online defines key as 'Of crucial importance' - 'she became a key figure in the suffragette movement', so it's a bit stronger than simply saying important. Other dictionaries also define it as very important, vital, crucial etc

I don't know its etymology, but keynote was being used to mean 'leading idea' as early as 1783. But it seems to me that several languages use key to mean important or essential, at least in certain expressions. In French, for example there's the expression 'position-clé':

Position-clef. "Le problème des transports (...) occupe une position-clé dans l'économie d'un pays" - L'Attif

In Spanish there's palabra clave = "palabra reservada cuyo uso es esencial para el significado y la estructura de una sentencia" - Diccionario R.A.E

"Key decision" seems to be literally translated in several languages (the noun for key is in brackets)

French - (cléf) - "La décision rendue par la Cour d'appel fédérale est une décision clé sur l'interaction entre les marques de commerce et les brevets au Canada"

Spanish - (clave) - "Una decisión clave: el futuro del Metro para Quito"

Italian - (chiave) - "Processo ai marò, rinviata a domani la decisione chiave"

Polish - (klucz) - "była to decyzja kluczowa" - it was a key decision

Czech - (klíč) - "Klíčové rozhodnutí pro digitální rozhlas" A key decision for digital radio

Maybe not key venue but something like it - key place:

"Pourquoi les bibliothèques sont un lieu clé pour une société numérique"

"El corcho y el alcornoque ocupan un lugar clave en la evolución del bosque mediterráneo"

"Warszawa jest dla has kluczowym miejscem w UE"

Key to mean important or essential seems to me make perfect sense and totally unexceptionable. In the British National Corpus, this in fact seems to be the most common use of the word key, after its meaning of 'solution, answer'. It collocates particularly with:

issue, role, element, factor, area, point, feature

This use in English appears to go back at least to the 1920s.

http://www.just-the-word.com/main.pl?word=key&mode=combinations

"The English language as it is being published in the press is crumbling around us" - Oh, I love that! Nonsense, of course, but great fun.

@Jasper - We're talking of a sort of indirect speech here, so need to think what he originally said to himself - If it was "If I move my arm it will break" (real conditional), then - He was convinced that if he moved his arm, it would break.

But if he was thinking "If I were to move my arm, it would break" (unreal conditional), then your sentence would be the way to go.

By using the "were to"construction instead of a simple past, you've already decided that this is a hypothetical condition and to use an unreal past, and therefore subjunctive "were" would indeed be appropriate. Whether you use "was" or "were", on the other hand, depends on how formal you want to be, and how attached you are to the subjunctive. Many people, especially Americans I think, would consider "was" wrong here, and as it's obviously from a piece of written work, "were" would be safer.

Whether to choose a real or unreal conditional here is another matter. Personally, I feel a real one would be stronger.

I've written a blog post on this and similar expressions for foreign learners:

http://random-idea-english.blogspot.com/2013/02/expressions-with-all-but-none-but-but.html

@Rory - On your second point, "all" is certainly synonymous with "everything" but it's anything but (except) synonymous with "anything"! Would you rather give somebody anything of yours they wanted, or everything of yours they wanted?

There is also the expression "anything but" which is very different in meaning from "all but", meaning "just the opposite". This example is from Cambridge Dictionaries -

"She's meant to be really nice but she was anything but nice when I met her."
= she wasn't at all nice, in fact just the opposite

We could compare that with:

"She's meant to be really nice but she all but ignored me at the party."
= she almost completely ignored me, i.e. she did everything except completely ignore me.

http://idioms.thefreedictionary.com/anything+but

You finish off by saying that you can't think of a single context to makes either word a synonym for "almost", and that's correct, they only mean almost when taken together, where "but" has its original meaning of 'except'. (Its more familiar use as a conjunction came into English later). And there are plenty of examples of "but" meaning except, including "There but for the grace of God, go I", which I've already quoted.

As camryn has explained, "all but" really means "almost completely", rather than simply "almost". And it doesn't really mean the opposite of what it suggests at all.

Martin quotes - “Such actions were all but unheard of then”, using camryn's explanation we could read this as "Such actions were everything except (completely) unheard of then" - i.e. mostly unheard of.

But in the end, I don't think analysing it too much is really going to help. Just accept it as an idiom. Nobody has to use it of course, and for many of us (older ones, perhaps) understanding it simply comes naturally. And it's useful to know when you're doing some reading :

"why he should change his mind when it was all but done, she could not perceive" - Jane Austen

"But as for Girton, the matter was talked over calmly, without either tears or kisses, and it was all but settled that Janet should go there in the autumn" - Charles Dickens

"She was all but worshipped by the peasantry around her" - Anthony Trollope

"that she's all but ready to fall to pieces in this same time" - Captain Marryat

"true, she all but consented, and did consent in a sort" - William Hazlitt

Admittedly, it's not that common, though. But then there's also the sub-idiom "in all but name" as in "They're married in all but name". - http://idioms.thefreedictionary.com/in+all+but+name

It's not really about 'between you and I', but there's an interesting Intelligence Squared debate on YouTube with the title of 'Between You and I the English Language is Going to the Dogs' between John Humphrys (BBC) and Simon Heffer (ex of The Telegraph, now at The Daily Mail) putting forward the motion, and Oliver Kamm (The Times) and Mary Beard (Professor of Classics at the University of Cambridge) opposing it - something there for all of us, perhaps.

http://www.youtube.com/watch?v=MMcSOCuszkw

But there are also admittedly examples where the co-chairs obviously assisted the chair, or, as in one 1959 example from the Music Operators of America (in Billboard), the Co-chairmen assisted the Chairman. And there's this example from the 2006 United States Code:

"The President shall designate a Chair from among the members. A Co-Chair of the President's Council of Advisors on Science and Technology who is not serving in the Federal Government and the Chair and Vice Chair of the President's Export Council shall serve as ex-officio members."

Whenever I (a Brit) see the word coworker (we prefer to use colleague or workmate) I think it's something to do with cows. Joking apart, I've never heard of this supposed difference, and that Oxford usage note AnWulf quotes makes no mention of it. At Merriam-Webster, for the prefix co- the definitions they give are:

'1: with : together : joint : jointly

2: in or to the same degree

3a : one that is associated in an action with another : fellow : partner

3b : having a usually lesser share in duty or responsibility : alternate : deputy

4: of, relating to, or constituting the complement of an angle '

They make no mention of the use or not of the hyphen. I can find only a couple of references to this on the Internet, one being your own Linkedin page. One university site says - 'Do not hyphenate prefix except when forming nouns, adjectives and verbs that indicate occupation or status.e.g. co-author, codependent', but nothing about a semantic difference.

As far as usage guides are concerned, Merriam-Webster makes no mention of it that I can see. Fowler's 3rd edition (1996 British) gives simply lists specific words where the hyphen is used entirely for practical reasons, nothing about semantics, for example:

co-respondent (for the reason AnWulf mentioned)
co-pilot, co-signatory, co-worker ('hyphen used to avoid momentary perplexity of the reader')
co-op, co-opt (clashing vowels might lead to pronunciation as 'oo'
New words which could be confusing.

In Britain I think we tend to hyphenate most where co- is followed by a vowel, unless words have become very well established. Oxford gives both possibilities for cooperate, although it's so well known that there is no real need now for the hyphen.

And as for 'cochair', I have to confess that it looks like Gaelic to me (in British books, co-chair is far more common). I don't know where you have got the idea from that a co-chair was subordinate to the chair - that would be a vice-chair, wouldn't it? I've just checked with three American dictionaries and haven't found that definition in any of them. The Free Dictionary doesn't list co-chair, Dictionary.com takes you to cochair. This is from Webster's New World College Dictionary (at Your Dictionary):

"co-chair
noun
a person who chairs a committee, meeting, etc. jointly with another or others

transitive verb
to preside at or over as a co-chair"