Well, I've watched No Country at least twice and I've been meaning to go back and watch There Will Be Blood again, so it's not like they're two movies that I only want to see once and then never again. I'd say No Country does defy conventions quite a bit, thanks in part to the story it's based on of course, by killing the main character halfway through and because of the ending. Just read people talk about it, a lot of people just hate, hate, HATE that, but I love it because you just don't movies do that very often.
There Will Be Blood reminded of the old epics from the 60's and 70's, everything's really large and expansive. But instead of being some great adventure or a war movie, it's about a guy who's completely despicable from start to finish and when you think he's finally lost everything at the end, he winds up getting exactly what he wanted all along: revenge against the only man that ever bested him. DDL's performance, just the overall look and feel to the world, and the final scene pushed it over the top for me. The scene where the oil rig catches on fire was pretty good too, especially with the music.
And even if these movies aren't necessarily breaking all the rules and turning everything up on their heads, they still require much more from the viewer in terms of concentration than a movie like Zombieland. With Zombieland, you can just sit back and watch the gore fly. You don't really have to think about anything at all, other than how cool everything is. I feel that movies like No Country and There Will Be Blood require a bit more than just that, and I appreciate that sort of thing.