I don't think there's a drop-off in quality, per se. The rest of the movie is still impeccably made and quite harrowing. But I think that it's easy to look at the Paris Island stuff as its own self-contained story. It's a high concentration of dramatic power that essentially pays itself off, so you've already released all that built-up tension by the time the movie actually gets into the Vietnam stuff. From there on out, the beats are more spread out and the movie doesn't dole out the big dramatic stuff quite as generously.

I think that for anyone who cared to do it, they'd find that Paris Island isn't the only episodic sequence in the movie, either. The whole thing is structured in vignettes. It's just that the rest of them are in Vietnam with a (mostly) different cast of characters.

I think the second half is manifestly worse. It's so unsubtle that I feel Kubrick is insulting my intelligence. Take the scene where Joker is at the Stars and Stripes briefing. Matthew Modine, in his "I am being a kidder voice" keeps underlining the absurdity of every statement and it's really, really poor writing and acting.

Lt. Lockhart: [reading] ... we have a new directive from M.A.F. on this. In the future, in place of "search and destroy," substitute the phrase "sweep and clear." Got it?
Private Joker: Got it. Very catchy.

The whole scene goes on like this. I get it. The media is trying to place an absurd, P.C. spin on things. I don't need your underlining there Mr. Kubrick.

I feel the same way about the Mickey Mouse Club song at the end. I get what you've done with the whole picture there Stanley -- don't need to put your cutesy bow on top

Kubrick has many virtues, but subtlety is not typically one of them.

I'm also not sure that subtlety is a quality that generally goes very well with war movies any ways.
-Jeremy