Something I didn't really notice the first time I read it, but has really struck me this time around, is how often Tolkien avoids directly showing anything resembling action. There are plenty of things that would count as "exciting action sequences" in the story -- but almost all of them happen offstage, with the characters in the narration either seeing distant effects and wondering what's going on (Aragorn and the hobbits in the Barrow Downs, seeing the lightning blasts from Gandalf's battle on Weathertop miles away), or they hear about it afterward (Merry and Pippin relating the fall of Isengard to Aragorn, Gimli, and Legolas). It happened in The Hobbit, too, upon reflection -- the death of Smaug comes to mind, or Bilbo being unconscious for pretty much the duration of the Battle of Five Armies.
It happens often enough that it can't be an accident. Obviously he's trying to set up an effect of some kind, but I am undecided in my mind what effect that is. And it isn't true of every action sequence -- Sam's fight with Shelob is shown in blow-by-blow detail, for instance. One would assume that the point of keeping the action offstage is because he wanted to emphasize the character of the people involved in the story, rather then get bogged down in the details of this fight, that fight, and the next fight. While I'd say you can learn as much about a person from how they fight as why, it's also true that you can get bogged down in too much of that stuff.
It may also be that Tolkien was trying to get across what it was like to be in a war, with battles raging in the distance while you hunkered down in your trench, worrying for your comrades and only hearing about what happened later. Or it may have been that he thought the "action stuff" was the least interesting or hardest part, and so avoided it when he could. (Hey, all writers are guilty of this ... SJ and NN have had a ton of things come up where I thought of some scene that I didn't want to fight with and so just sorta skimmed over.)
The other thing that strikes me is how much easier it is to follow the story, now that I've seen the movies. And how easy it is to see where Peter Jackson wandered off into his own fancies. Overall, I'd say he did a pretty good job translating a very talky book into a visually-compelling and exciting movie -- although I still think he really bungled on poor Faramir.
Oh, and Gollum tricking Frodo into sending Sam home -- and Sam going? No.
No, no, no, no, no, no, no.
You need to redo that bit, Peter.