9It is entirely possible to use this idiom in a positive context: "Yes, quantum field theory really is that difficult of a subject." – Aaron Rotenberg – 2017-09-12T01:10:40.780

5@AaronRotenberg but still incorrect, the correct Englsh form would be "Yes, quantum field theory really is that difficult a subject." – Mick – 2017-09-12T02:28:14.700

7@Mick It's almost like idioms don't always make literal or grammatical sense. – Fund Monica's Lawsuit – 2017-09-12T04:16:11.427

2@QPaysTaxes: I think the point is that, in many parts of the English-speaking world, the "of" is not idiomatic and is regarded as ungrammatical - certainly where I come from (southern UK) we don't say that (we'd say "it's not that big a deal" rather than "*it's not that big of a deal"). – psmears – 2017-09-12T08:15:02.600

I think 'of' here is just wrong, like when someone says "you should of cleaned the car" instead of have. – bye – 2017-09-12T09:45:49.253

@DrEval Those aren't even remotely close to being comparable meanings of "of" in this context. One is the clear misuse of a word when another one is correct, while this is more a case of inserting an additional word into an expression. It's technically redundant, but not inherently incorrect. – John Clifford – 2017-09-12T12:10:26.100

@psmears ...and? I don't see how that contradicts what I said. They're idioms; those are regional things, and [insert first comment here]. – Fund Monica's Lawsuit – 2017-09-12T12:55:21.200

1@QPaysTaxes: No, not all idioms are regional things, not at all. There are plenty of idioms that form part of standard English. Mick was pointing out that this one isn't standard (which is a very useful fact for learners of English); you seemed to be objecting to that. – psmears – 2017-09-12T13:23:23.300

The idiom is not required to be inverted. It can also be played straight. – godskook – 2017-09-12T13:56:19.017

@psmears ...huh. I never knew there were idioms that were part of the language; my understanding of what an idiom is is a regional (or just tied to a certain dialect) phrase that may or may not follow the normal grammar rules, but has a certain accepted meaning. In other words, it's perfectly understandable to have nonstandard, grammatically incorrect phrases that are still used, because they're... Idioms. And the ones that are tied to a dialect/region might, obviously, not be present in other dialects/regions. – Fund Monica's Lawsuit – 2017-09-12T16:36:46.370

7It might be worth noting that "Is it really that hard of a concept" is very likely to be encountered in informal contexts, although it is deprecated in formal registers. – StoneyB on hiatus – 2017-09-11T14:22:29.693

2There are good constructions that violate your rule: the first of a series of examples, the best of the films I have seen, etc. I imagine that these doubtful constructions, like "that hard of a concept to grasp" are based on false analogies. – Chaim – 2017-09-11T17:53:29.317

3Note that the link doesn't say that "adjective of a noun" is strictly incorrect. It says that an adjective there is perfectly fine in standard English if the adjective is one of quantity (e.g. "too much of a drive"). It's only adjectives of degree that are proscribed. -- They also go out of their way to say "it shouldn’t be called incorrect—just inappropriate in formal English." – R.M. – 2017-09-11T18:22:54.423

1@R.M. I do not understand the difference between incorrect and inappropriate. All language usage is determined by custom, not logic. Whether one usage is correct or appropriate or standard or not depends on the degree to which it is used. – sammy gerbil – 2017-09-11T19:21:25.670

@sammygerbil And I don't understand what you mean by "not strictly correct". Isn't it more important whether something is or is not incorrect than whether it's strictly correct? (And, as far as I can tell, nobody is arguing that this is incorrect.) – David Schwartz – 2017-09-11T22:42:35.453

@DavidSchwartz What I mean is that correctness of language is a matter of the extent of usage, ie what is accepted as standard among the population. So by "not strictly correct" I mean that this usage is in the minority or it is widely used but accepted as not being officially correct. – sammy gerbil – 2017-09-11T22:59:54.180

1@DavidSchwartz "And, as far as I can tell, nobody is arguing that this is incorrect." - "Incorrect" or not, I've never seen or heard it used in the OP's context in literate British English, in the last 60 years. Maybe it's "correct" in some US dialects of English, but I don't know about that. – alephzero – 2017-09-12T00:52:12.583

2

I like that this answer discusses the regional/dialect-specific nature of this construction. It would be a good answer if it replaced the "correct"/"not correct" labelling with more precise "formal"/"standard"/"written" qualifiers. It's common enough in spoken American English that most people wouldn't bat an eye at it; in COCA, it's actually more common in their spoken corpus since 2005 ("not that big of a" vs. "not that big a").

In any case, in phrases such as @R.M.'s "too much of a drive", the words before the "of" are a noun (phrase), not an adjective (phrase). – Rosie F – 2017-09-12T07:23:19.400