>Also, I thought that Yudkowsky's HPMOR fanfic had more interesting ideas than the whole Rowling series, which I like a lot.
Then you do not understand writing. If Yudkowski really had more interesting ideas, then he would have been able to do HPMOR as original fiction.
Rowling is actually really good, inventing very charming things, very fun sentences, and there's nothing even close in HPMOR (I have read it myself, and enjoyed it to some degree), but you really underestimate how good Rowling is.
Yes, pedantically, and as mentioned in the Notes in the Text in most editions, the Lord of the Rings is a single book sometimes published in three volumes.
Thus it can tend to become limiting; and I say this as someone who actually does enjoy fanfiction.