Those Little, Unimportant, and Unknown parts that we love...

[Guinevere]

I agree with you, zb, I am also very fond of the "archaic" language Tolkien uses: and I love all the alliterative verses!
I think his use of different styles adds to the feeling that the characters appear so real and believable and "historical". It also produces what Tolkien once called "the heartracking sense of the vanished past."

But what I like especially, (and hadn't noticed when I read it the first time..!) are all those "proverbs" and wise sayings that are everywhere in Tolkien's work. Things that a character says that relate to the situation in the book, but at the same time convey a general and timeless meaning.
Once I had become aware of it, I keep finding more and more of those! I think Tolkien must be quite unique in this respect!

eg: "The wide world is all about you, you can fence yourselves in, but you cannot forever fence it out."
"Faithless is he that says farewell when the road darkens"
"Faithful heart may have foreward tongue"
"Of evil will shall evil mar" "Oft hope is born when all is forlorn" and so on (See also the Gaffers mixed-up proverbs game on the Main page of the BD!)

One of my favourites is what Haldir says:

The world is indeed full of peril and in it there are many dark places. But still there is much that is fair, and though in all lands love is now mingled with grief, it grows perhaps the greater.