LOTR on the other hand is almost purely light-hearted stuff.
Huh. I wouldn't describe it that way, even if you're going only by the movies.
Oh, it totally acts like everything is really serious, but that's about it. It's almost all goblin/elf/hobbit/magic stuff, there's no be-headings or kids getting pushed out of towers or anything even close to that.
So ... gratuitous violence and sex means it is more 'serious'?
I'm sorry, but I think that just makes them more emotionally manipulative. Those are simply plot devices used to get visceral reactions out of viewers/readers. They don't make a story any more 'serious' than a story that has no violence, no gore and no sex. There are plenty of crappy books and movies that aren't 'serious' at all that are filled with blood, gore and sex.
Note that just because I'm accusing GOT of being filled with gratuitous plot devices doesn't mean I think it's bad stuff. Those elements are part of Martin's toolkit and he at least is using those devices to carry his plots forward ... hopefully somewhere! For a hack writer, there would be no real plot behind such devices. Martin, at least has real plots going on. Of course, until he wraps everything up, the whole thing in the mean time is just soap-opera. Like a Marvel Comic series. One shocking scene or surprise twist or cliffhanger leads to another. That's not a bad thing. But it's not necessarily great, master storytelling.
As literature and great storytelling, the fact that Tolkien relied so little on such tools is testament to the greatness of his literary skill, his craftsmanship and the power of his story. And Jackson's films, I believe, do a great job of carrying that forward. My only complaint about the films is that they tend to get maudlin and more than a little campy at times. I could have done without Sam and Frodo staring wist-fully at each other and droning on and on (something I didn't really get from the books).
I like them both. I've read both (still waiting on Martin for those final books....). I've seen all of both HBO's GOT and all the Jackson films. I admit I've only read through Martin's books once, whereas I've read through the LOTR books multiple times over the years.
Imho, GOT is a fun romp. As a TV series, it is definitely ambitious and in some ways ground-breaking (but as a drama series, imho it pales behind some of HBO's other efforts like the Sopranos, Deadwood and Six Feet Under).
(Aside: GOT fans might also enjoy Michael Hirst's
Vikings series, which gets shown on the History channel. Same sort of complex plot lines, complex blending of good-and-bad in characters and always the sense that an axe or sword will fall across any character's neck at any time...)
But LOTR is imho epic, both grand and subtle and above-all, thought-provoking. The books are a literary classic of the most deserving kind and the films, though not perfect, are standard-setters for bringing such a giant literary work of fantasy to the big screen.