Okay, here we go.
People who defend Tolkien try to cite "product of his time" and "mythical epic style" as their bases, but this fabled time must be something rather different from the twentieth century I know of: writers who were his contemporaries--and even writers who lived and published and died before he--were perfectly capable of writing concise prose. Dismissing people who don't like Tolkien as simple whiners who want modern action-packed thrillers is nonsensical, polarizing and rather sad. Do you want to praise his precious landscape Romanticism? But it's been done before, and so much better, but even if not that's a strain of writing that's so inherently insipid that it parodies itself. It clashes intensely with the attempt to be epic and mythical. If anyone's read Old English or Old Norse texts, even in translation, you will notice a distinct lack of elaborate, wordy descriptions. I don't have my copy of The Poetic Edda with me, so you'll have to bear with a random translation of Atlakvida:
"Thou giver of swords, | of thy sons the hearts
All heavy with blood | in honey thou hast eaten;
Thou shalt stomach, thou hero, | the flesh of the slain,
To eat at thy feast, | and to send to thy followers.
"Thou shalt never call | to thy knees again
Erp or Eitil, | when merry with ale;
Thou shalt never see | in their seats again
The sharers of gold | their lances shaping,
(Clipping the manes | or minding their steeds.)"
...
With her sword she gave blood | for the bed to drink,
With her death-dealing hand, | and the hounds she loosed,
The thralls she awakened, | and a firebrand threw
In the door of the hall; | so vengeance she had.
Gudrun feeds her children's flesh to her husband before proceeding to murder him and burn his hall. She's not condemned for this: if anything, this is considered as right and dutiful, since as the last member of her family, it falls to her to take revenge, female or not. Next to her, Eowyn is a suckling babe.
Compare that to... well, anything from Tolkien. He draws from Norse and Old English sources, which are terse Germanic things, powerful and ruthlessly efficient in their scantness. In short, everything Tolkien's prose isn't. He tries to shoehorn Germanic warrior code into Rohan and even the grisliness of The Siege of Jerusalem (and if you don't know what I'm talking about, then you aren't as well-versed in this as you'd like to think you are, are you?) into the siege of Minas-Tirith. Except without the unapologetic brutality. He does what Disney does to fairytales: sanitize and water down until there remains only vapid family-safe sludge. Those things were already reinterpreted and refitted for Christian sensibilities by silly monks, and they hardly need a second dumbing-down to fit a bloodless, New Testament paradigm.
Another point is something most Codexers won't see anything wrong with: Tolkien's an ethnocentric bore. Notice that I don't use the word "racist" because I'm sure he held no active, conscious opinion of non-English people, let alone participated in bigotry (whatever else people might shout about "orcs are black but humans and elves are white!"). It's not so much that I find his setting unoriginal because I've already read other fantasy; it's that I see absolutely nothing in the so-called beauty of Middle-earth. It's dead to me. I have no patience for a dead white man lamenting the tragic loss of gray, insipid countryside in the most self-indulgent way possible. It's so hilariously, incredibly privileged (let all know their places, let Sam always call Frodo "master" because we can't have servants and country gentry be equals, and shall we get started on his attitude toward industrialization and progress?) that I can't take it one bit seriously. And Tolkien takes his work terribly, awfully seriously--apart from the flashes of weak humor here and there, it's all a long parade of serious business written in absolute earnest. There's no room for anything else but this grim march toward trumpeting epicness. You're either swept along, or you're left feeling nothing at all because the melodrama just doesn't work for you.