I found it very hard to write down my thoughts on this film. Like previous films who were clearly my most awaited cinema experiences of the year (e.g. The Matrix Reloaded or The Fountain ) I was quite disappointed, though I fortunately enjoyed the film and found much to be liked. I didn't think that Day-Lewis gave the best possible performance (though he certainly was great), and I actually preferred watching Paul Dano (possible Oscar for best supporting role imo) and also Dillon Freasier (as Plainview's "son"). 
Besides that, I thought that either I didn't understand the film, or the film was trying much without ever reaching it, because what it was trying to say was very simple, yet disguised as a "grandiose" epic. Perhaps an ironicaly playfull and self-conscious attitude, but I doubt that Anderson has started to follow the Coens' footsteps in this regard. Like R6dw6C I also need a second sitting, but i doubt that I'll ever think of the film as an " instant classic" or a masterpiece. I like it very much though, but less for what it's saying about America (which I think is only a small part of the film and even moreso Anderson's intention), than about the portrayal of its characters in a desolate world. Griping for a way out, some sort of humanity - as always in Paul Thomas Anderson's films. Same as usual one might say, only that Anderson progresses in terms of direction in a way that most other "mainstream" filmmakers can only dream of. He is truly doing leaps from film to film that would take decades for other directors. I probably still prefer his direction in Punch-Drunk Love (2002), though I'll probably have to admit that he HAS again gotten much better. It seems that the simple days of mimicking other directorial styles and too "orderly" compositions (like in Magnolia (1999)) are over once and for all. This definitely earns my respect. 

So far from what I've seen I would rank Anderson's films this way (the first two are personal favorites): 

1. Punch-Drunk Love 
2. Magnolia 
3. There Will be Blood 
4. Boogie Nights 

Nevertheless I think in terms of artistic vision ( though not the same as "execution") they are very similar, and are (thus?) also difficult to separate in terms of quality. Great is great is great. 

As i can't seem to find the right words, here's an interesting review of the film which expresses some of my views very well (especially the first page). But I don't agree with the overly simple conclusions that are drawn at the end of the article and I'm also not in line with the dismissal of the film on (merely) the terms stated by the author. 

Stephanie Zacharek on "There Will Be Blood" 

PS: I watched the film in English without subtitles on a huge theater screen during the advance of a bad flu. That may explain some of my problems. LoL