I haven't fully researched this out yet, but as one of those grain-of-sand-type irritants, it's worth recording.
Sometimes when I wish to amuse myself, I go and read some of the numbered list articles at Cracked.com. Here's one from 2010 that I just came across: "6 Great Novels that Were Hated in Their Time," and by "hated" they mean "hated by critics and readers alike when they first hit shelves."
Well, maybe not.
I've read all six of them. One I found merely uninteresting, two in my opinion deserve every negative review they ever got (I'll let you guess which two), but the other three really hit me strongly when I read them at a tender age, and I still consider them masterpieces of their kind.
But were they hated? Steinbeck's The Grapes of Wrath, for instance, did get a lot of political criticism from the right when it appeared in 1939, but not so much critique for its literary qualities. It was a huge bestseller, majorly talked about, and that successful movie adaptation the write-up mentions came out only nine months after the book was published. That was really hot stuff, even then.
And then, at the end of the list, comes The Lord of the Rings. The entry quotes the Drout Tolkien Encyclopedia as saying that "No 'mainstream critic' appreciated The Lord of the Rings," but besides the fact that this comes from one of the less reliable contributors, what it seems to mean is more that they weren't in a position to fully appreciate its greatness, not that they didn't like the book. Because in fact, while the book did receive its share of severe pans (most famously from Edmund Wilson), it also got many strong positive reviews (most notably from W.H. Auden and Naomi Mitchison). It also got readers, selling remarkably well for a three-volume novel of a highly unusual kind from a basically unknown author. Most of the really hostile critical commentary on Tolkien dates not from when The Lord of the Rings was new, when the kind of people who didn't like that sort of thing mostly ignored it, but from more recent years when its popularity has become massive.
Among the negative reviews the article quotes is one from The New Republic describing it as "anemic, and lacking in fiber." That puzzled me, since The New Republic's review, which you can read here, concluded with unsurpassable enthusiasm, "There are very few works of genius in recent literature. This is one." (A famous line, given that it was quoted in innumerable blurbs for decades afterwards.) Did it also call the book "anemic, and lacking in fiber"? It did not.
I had to do a little digging, but it appears that this quote comes not from any review when the book was new, but from a New Republic article - called, with oh so brilliant originality, "Bored of the Rings" - from January 2002. That's right during the hoopla attendant on the release of the first installment of Peter Jackson's movie adaptation a month earlier, and not far in the wake of all the mass polls declaring Tolkien's book one of the most popular of its century. So, rather than a critic not recognizing a new and untried book as a masterpiece, this is someone spitting in the wind in defiance of a long-established masterpiece.
There's more. In the context of "mainstream critics," calling Isaac Asimov a "heavyweight" is laughable, even before knowing that Asimov denied any critical ability and hardly ever wrote book reviews. The quote in any case is not a quote from Asimov, but a paraphrase quoted from the same unreliable article as the line about the "mainstream critics," and while Asimov might have said something of the sort as a cautionary note, he was actually a great admirer of The Lord of the Rings, which he read several times. (Interestingly, Tolkien himself once named Asimov as an SF author he liked, so there was a mutual admiration society there.)
Since I've actually been compiling together a list of early reviews of Tolkien - it turns out that each of the standard bibliographies has items that others missed - I ought to measure also their grades of the book, and see how it came out. But I do know that a lot were favorable.