There's a lot of really cool stuff going on in this movie, but the directing and writing gets in the way. Ultimately, the movie tries to link the Viet Nam war and racism, and fails miserably. In trying to fight the two different wars, it loses a lot.
There's a lot of issues with the movie, but they come down to three issues:
1) It needs to decide which movie it's remaking. It basically tries to combine Treasure of the Sierra Madre, Apocalypse Now, and Three Kings and fails at it because of how different the three movies are.
2) The length; this movie could have been told much more effectively in almost half the time. I know it's trying to combine three movies, but that's no excuse for just taking too much time on things.
3) The over-the-top racism. I know this is a weird call, but the movie could have been a lot more effective had it toned down the racism. Also, some of the incidents involved contribute to the length of the movie without really adding anything.
The apparent lack of any military knowledge on the part of Lee (the sheer number of military gaffes, such as salutes and ammunition) as well as the obvious call-backs to movies involved keep throwing watchers out of the movie.
This had the potential to be an incredible movie, especially as the acting is incredible and the cinematography is gorgeous, but it's just too long and too many issues to be even a satisfactory movie.