A correspondent writes to say that he caught himself saying All I find is idiots, and wondered whether it shouldn't have been All I find are idiots. He was saying to a friend that, when he wanted to find an expert in a particular thing, all he found was/were idiots, and he goes on: 'I realize that if all is used with a plural noun (with or without a partitive genitive) it takes a plural verb, i.e. All (of) the men are idiots, but what happens when all is used on its own? Is it like what, which, according to certain grammarians (Fowler, Gowers), can take a singular or plural verb? Or does it always require a plural verb in sentences such as my outburst?'
Certainly all on its own can be found with either a following singular or a plural verb.
All are welcome
All know the answer.
All are idiots.
All have a role to play.
Usage usually has the all reinforced with a pronoun, such as We all know...
All has gone quiet.
All is not lost.
All's fair in love and war.
All is dreams and sexual madness for Strindberg. (corpus example)
What we have, in other words, is a distinction between countable and uncountable. In (1) all could be replaced by many people; in (2) it could be replaced by everything. As my correspondent's example is plainly countable, the normal usage would thus be All are idiots. All is idiots is ungrammatical in standard English (though I've come across it in some regional dialects).
So why did my correspondent say All I find is idiots? I think it's because the postmodification I find alters the semantics of the subject, allowing an uncountable interpretation. He is right: it is pulling the construction in the direction of what. Fowler spent over two pages in his Dictionary of Modern English Usage trying to decide which way to go. He corrects, for example, What I wish to point out are certain instances... to What I wish to point out is certain instances.... He does not actually have a separate entry on all, but I imagine, if he had, he would have treated All I wish to point out... in exactly the same way.
So my correspondent used the singular because, quite plainly, all the so-called experts out there were being perceived as varieties of a single species, 'idiot'. If there had been a singular uncountable noun to express this notion in English, he would probably have used it. (Compare: All I see are leaves vs All I see is foliage.) But there isn't one, without saying something like All I see is idiocy. The reason he (and Fowler) feels uncomfortable is that there is always a problem when subject-concord concord pulls the verb one way and complement-concord pulls the verb another way. Usage has been split on this one for two hundred years, at least, and doubtless will continue to be, depending on whether the speaker has a unified or diversified meaning in mind (cf. committee is vs committee are and many other examples).