Evidently, no one who ever lives in naked guy's apartment ever closes the shades (except for Rachel's sister, but she doesn't count since she wasn't living in the apartment). Monica and Chandler having sex, Rachel dancing seductively. I'm sure New York City has seen more of the friends than most people care to see.
The running "Chandler and Monica having an affair" storyline was the funniest ongoing arc in the series' history, and this episode, the climax of it, was one of the all-time funniest.
I haven't seen most of the rest of the arc, but I particularly enjoyed the subplot with Ross trying to get Ugly Naked Guy's apartment. Particularly these scenes, in particular (and paraphrasing):
Ross (looking confident): I decided to drop Ugly Naked Guy a little bribe...
(Gang go to window to look into the apartment)
Monica: Wow! Was it that huge pinball machine!
Ross (thunderstruck): No...
(a sequence in which someone asks if it's another big expensive present and Ross says no - can't remember the details)
Joey: Well, then what was it?
Ross: The basket of mini-muffins...
Rachel: But there are hundreds of baskets of mini-muffins there! Which one's yours?
Ross: The...small one.
(Great example of farcically making a situation worse and worse).
And later:
Joey (looking out of the window): Hey, Ugly Naked Guy's got a naked friend!
Monica: It's naked Ross!
(Particularly because this ploy actually works!)
GREAT LINES:
"Im so glad that we're going to be having all the sex!"
AND...
"GET OFF MY SISTER!!!!"