The best things: 1. Chandler and Ross are really good characters 2. They explored a lot of subjects with the stories and the show never grew stale 3. There is a large number of episodes that are absolutely stellar and a small number with no redeeming qualities (I just watched the one where Chandler gets stuck with a supermodel on an ATM machine room during a blackout, it's hilarious).
I must point out the 3 worst things, though: 1. Overly stupid characters like Joey are irritating and the worst type of characters in sitcoms, ever. 2. The female characters are weak. Phoebe is awful most of the time, and Rachel and Monica became a one-trick phony after season 6. 3. The Ross-Rachel story was incredibly well done during most of the show but the series finale was disappointing.
I agree with most of this, but while Phoebe character's had no depth (except for always alluding to her sordid past), she was always the corny, funny one of the girls.
Totally agree about Joey. Like I said before, they made him more stupid over time, which make him less likable and believable. Granted, he still had plenty of funny moments later in the series (JOEY DOESN'T SHARE FOOD!), but he also had tons of cringe-worthy ones as well.
And I definitely agree about the Ross and Rachel romance. I had a few girl friends who thought they should have always been together, but they were funnier on their own, being single, and having interactions and whatnot. Okay, Rachel had some disasters later in the series (Tag, Joey, etc.), but Ross had so many hilarious experiences with dates, and that wouldn't have been possible had he and Rachel been together all along. Not to mention that once Chandler and Monica became a couple, you didn't want to have two couples out of the six friends.
That leads to another question: was it always planned all along to hook Monica up with Chandler, or was that thought up on the fly? I thought it was genius at the time. It was something no one saw coming.