When did the show really take a turn for the dark side?

Just now!

I never watched a lot of this but with Adult Swim re-airing it right now I've gotten the chance to see a lot and I had no idea the show took a very different direction. It's still occasionally very funny, but it's so much darker.

I've been able to look forward to unseen episodes like new ones every time they're on, now I'm going to start paying attention to what time I should be tuning in.

Aw, man. There's too many to mention.

The EXACT moment the show turned into what it became in Season 3 was in Nature. The second Clay said "Who am I kidding? I hate myself." You could actually feel the entire tone of the show changing the very second it cut to Orel's reaction.

The show was always spot on with its cynical and sarcastic humor in the first two seasons (Dino, of course, no big love towards the church and hypocrisy in general), but there were multiple occasions when the show would cock its head sideways and say, "There's something else here too." It's these sideways glances that gave the comedy more depth, a depth which was sadly under appreciated.

Here's a list:

"Love" - I don't know how anybody can consider the ending of this episode to be funny. Now, Bartholomew carrying the cross-shaped stick up to the mountaintop on his way to be euthanized was worthy of a chuckle, but when Bloberta smashes Orel's hopes of meeting his dog in heaven by telling him dogs have no souls, the episode stops being funny. That's just...sad.

"Courtship" - the whole episode, especially the ending with Doughy sitting outside his own house while his parents get it on inside.

"The Best Christmas Ever" - Well, duh. Episode stops being funny about three minutes in and it ruined my Christmas the first time I saw it. I think the crowning moment of serious business in the episode is the "morning after" scene with Bloberta just sitting in bed. No music, no dialogue - just a series of shots that culminates in her shedding a single tear.

"Maturity" - "I GAVE MY WHOLE LIFE TO YOU!" *smack* Hell, half of the episode is devoted to dropping hints that the adults in town are even MORE screwed up than the show lets on.

"The Lord's Prayer" - the Shapey/Block switch was already mentioned, but I think I'm going to mention one of my favorite parts in the whole series:

"Still hate her. *gulp* Still hate her. *gulp* Still hate her. *gulp* Toler...hate her. *gulp* Tolerate her!" A private moment that sums up everything we need to know about Clay going into Season 3.

I thought the final season was brilliant and one the best final seasons of any show ever it wrapped up the series perfectly. i just wish there was maybe one more season in between the second and third to make the transition a little smoother....either that or i just wish there were more episodes. anyways one of the best series ever and the if you watch the third season all together in order it's amazing.

I thought it made it better. This may have been the first show of its type to make me get choked up. Also I think it's the first show on [adult swim] to have real character development.

I loved it all towards the end: the exchange between Orel and Rev. Puddy where he gives Orel some REAL positive affirmation; Orel's dad dealing with his issues; the final episode that shows Orel growing into a good man.

So yes, lot's of the funny was gone and no it doesn't really fit in with the [adult swim] pot smoking shows, but that's what I love about it.

To answer your question though, I think it was mid-season 2 through season 3 that really changed the tone. Once Orel's mom and dad started drifting towards a divorce.

Like all shows, it ran out of gas.

Actually, I would say pretty many of the episodes BEFORE "Nature" was very dark too...

"Loyalty"
"The Best Christmas Ever"
"Love"
Yes and no.
The episode is extremely funny and the dark comedy is handled lightly, but at the same I get ultradepressed when watching the ending...
"The Lord's Prayer"
Surprisingly darker than even most episodes in season 3.

The rest of the episodes though are pretty much just extremely funny.