If you want some friendly advice, get a haircut and take a bath. You wouldn't get hassled so much.
-Teasle

One of the greatest action movies of all time. It's gritty, incredibly dark, has an air of realism and is still a very original film to this day. I say an "air of realism" because even though some things might be unrealistic when you think about them too much, the movie is made in such a way that pretty much anything John Rambo does comes off as totally possible. That's how you make an action movie, you don't just do the impossible and call it a day. You make the impossible seem possible and THEN call it a day.

I say the movie is original to this day because unlike most action movies that have the clear cut good guy, battling the clear cut bad guys, this movie is very grey in that area. John Rambo is hardly a clear cut hero. He basically has a violent mental breakdown in the film. The cops aren't exactly the big bad guys from most action movies either. Their biggest crime (most of them anyways) is being a bunch of assholes really. You enter a grey area like this supported by great actors and you have one hell of a ride on your hands.

The character of John Rambo in his first outing is an incredibly dark character. I don't think Sylvester Stallone gets enough credit for his performance in First Blood because when people think of Rambo, they think of all the films combined. When you sit back and only look at this film, he gives an incredibly nuanced and subdued performance as the damaged Vietnam Vet. Some people might think I'm exaggerating with that, but that's my honest opinion of his performance.

The other standout of course is Brian Dennehy as Sheriff Will Teasle. His performance really is the next best one in the film, but it's not because the rest of the cast sucks. On the contrary, the entire cast in the film give very realistic performances that is uncharacteristic of an action movie, especially in the 80s.

The violence in the film isn't actually gory or over the top as most people remember, those of us who watched the film back in the day anyways. Because of the tone of the film and the performances of the actors actually playing characters, when violence does erupt it seems much more disturbing and real. When Rambo holds up his knife to Teasle's throat, even though I know he isn't going to kill him, I still wince. In Expendables when Stallone chops off "Guard with no name #32"'s head off with his knife I feel nothing.