Let's consider this purely as music, and leave aside the questions of style/instrumentation or how well the music supports the actions on the screen. Although there are issues with those aspects as well, the main problem (and it's definitely a problem) with the LotR sound track is that it's the musical equivalent of a clip show. The visuals of the movie help distract from this, but if you start listening to it by itself, the themes-pasted-together aspect becomes pretty, well, obnoxious.
Most music--most music that gets listened to by people who aren't music theorists--has forward motion. It's a bit like story-telling, that way. While there may be repeated themes (fugues are nothing but this), they aren't simply stuck next to each other; the music has to develop and build, and turn the themes inside out and upside down, change tempo and key and harmonization.
I'm not saying there's none of this in the LotR soundtrack, but there's not a lot of it, either. Having listened to the TTT soundtrack on repeat for two days at work, I feel I can say this with confidence. Especially since this is the album, not the complete soundtrack--this is what's cleaned up for casual consumption outside of the movie. Not to mention I started listening to the collected soundtracks while writing this and had to give up because it was so jarring I keep losing my train of thought.
Maybe that's a good image to use: if you think of music like a train, with different cars all connected and those connections being the thread that carries through, pulls the whole thing along, much of the LotR soundtrack is like cars which are placed next to each other but not actually attached. (Okay, that sounded better in my head.)
Or, as a story has narrative threads holding it together, so too does music. Without those threads (and this is what I hated about trying to write papers in sophomore music class; I don't have the right vocabulary for it) you don't have a story, you just have a bunch of disconnected scenes. Without musical transitions and arcs and whatnot, you don't have a piece of music--just musical phrases laid out in a certain order.
Okay. Maybe it'll make more sense with an example or two. I'll start with this one, since it's what kicked this whole discussion off:
You've got the little intro bit up to around 0:20. Then the generic elvish chanting up to 0:42, which is when the sort of Enya/pop-singing starts. This actually carries along fairly well as a cohesive piece of music, with some back-and-forth between singer and orchestra, but at 2:10 or so there's a very audible disjuncture, as we're abruptly in one of the Shire themes (if I remember correctly; I don't feel like pulling the DVDs out to check). At each of those time markings (approximately; I'm probably off by a couple seconds) you could cut the music and not lose anything. The musical thread stops and another one starts, and there's no connection between the two except a lack of auditory space.
This is a fairly mild example--some "pieces" from the soundtrack nearly give me whiplash, with a different theme every 30 seconds.
Okay, here's another one, which is an example of the reverse issue: no themes, just generic "spooky/anxious" music. Chop it up into two- and three-measure bits and rearrange as you please--the listener wouldn't be able to tell the difference.
And yes, I know these are just a few minutes out of hours of music, but these are random samples. I typed in "LotR soundtrack" and grabbed the first ones that were less than 5 minutes long. Just try listening to the whole thing without noticing what I'm talking about.
So, counter examples, from a couple of movies with soundtracks I really like.
Sherlock Holmes (in context) -- the music in this scene is what sold me on the movie. A great example of how you can use silence without losing forward momentum. Actually, this movie is also a brilliant example of how to use and reuse the same theme over and over without it becoming distracting or repetitive. I spent an afternoon listening to it on repeat, and was consistently interested in what it was doing.
Amelie (in context) has gorgeous music--if you didn't know it was from a movie, you probably wouldn't guess.
O Brother Where Art Thou takes completely separate songs and fits them together so that they flow from one to the next--if you want to make a movie with songs without turning it into a "musical", this is your template. (Which we'll probably come back to if I ever write up my theory of how the LotR soundtrack should be done, given how much singing there is in the text.)
The difference here is that the music is more than just musical cues: in each case, it has its own integrity, its own shape and motion. You can listen to it as music in its own right.
... And I feel like I should have an actual conclusion here, but you'll have to forgive me. It's been a while since my essay days, and I'm still trying to remember how to do this properly.