Terrence Malick's Badlands, Days of Heaven, and The Thin Red Line are among my favorite movies. Humorless as they can be (though Badlands is pretty funny in a horrible deadpan way), they're also extraordinarily beautiful and earnestly spiritual. They're prayers of thanksgiving in which Malick works through the obvious horror of life to articulate what there is to be thankful for. He's a sentimental realist, spiritually articulate, fully alive to the compelling arguments in favor of a nihilism that he works hard to reject. All his movies might be about the friction between "nature" and "grace," though The Tree of Life is the most explicit.*
I thoroughly enjoyed and appreciated The Tree of Life even when it doesn't quite work for me, especially in the final reel, in which Malick attempts to weave a visual coda from all the associations the previous 'stanzas' of the movie have laid down and articulated; though in fairness, this kind of symbolism is tricky as nitro even in hands as steady as Malick's: the associations and images can be fragile containers of the meanings they've acquired. The movie opens with a quote from The Book of Job, and Job is referenced at least twice more in the film's sparse dialogue. It helps a lot to have actually read The Book of Job rather than merely be 'familiar' with it. For long sequences, what we see on screen is the visual equivalent of "the Voice from the Whirlwind," which in The Book of Job upbraids Job for his capitulation to despair. I recommend the poet Stephen Mitchell's translation, which, back in my late twenties, during a 11,000 + cross-country car trip that took me across Canada, to southeastern Alaska, down the west coast of the U.S., and back across the mountain west, was the catalyst for one of the great epiphanies I've experienced, one which I value all the more as I find that epiphanies come less often as I grow older.
I also love how Malick layers sounds on top of images, e.g., sounds of waves crashing over images of stellar nebulae uncoiling. Also watch for how he moves human beings through the frame. They're always in motion, one stepping into background as another steps into foreground, etc., so that characters are always telling both their private story and relationship stories visually, if not in dialogue.
Lastly, though many viewers have complained they couldn't follow the 'plot' in The Tree of Life, I had no trouble. All one must do is be willing to hold a question in mind for a longer time before it's answered than most Hollywood movies ask viewers to do. I admit to enjoying the sensation I experience by not having the answers too quickly: left open, they create a space for images on screen to acquire meaningful associations with the human story that frames them: if plot questions were answered too quickly, images on screen would appear to be merely random. I find that this movement between still-open plot questions and (otherwise unrelated) images on screen to be soothing, particularly since the human story of the film is as elemental as it is, drawing on our childhood memories of our mothers and fathers, limning the origins of our life long struggle to choose grace, while working to survive in nature.
(*Oops. I say this not having seen The New World.)