Nickelodeon Doug vs. Disney Doug
It's pretty frustrating to see that all that most people can spot about two series that are as different as night and day is stuff that the writers pointed out in the first episode of the Disney Series. "Uhhh, Roger is rich now. Uhhhh...The Beats broke up. Uhhhh...the Honker Burger was bought out by some French thing. Uhhhh...they drew the characters differently."
Most of those were just story-driven changes. The Nickeldeon Doug could've done all those things (well, maybe not the Roger thing) and the show could've been just as brilliant.
The important difference is in the structure, the writing, the acting, and the art work (not how the characters were drawn - I'm talking about the background and prop art).
1. We'll start with the structure, which affected the writing and structure of said writing. The Disney stories were 22 minutes in length, as opposed to 11 minutes each on Nickelodeon (the 3 exceptions to that being Doug Bags a Neematoad and the 2 holiday specials). This led to a lot of padding and fluff, which we'll get to in point 3.
2. Rather than telling the entire story through Doug's narration, with the entire episode being about Doug and his role in whatever conflict arises, which is how the entirety of the Nickelodeon series was told, Disney's Doug has the structure of a sitcom or soap opera, showing different characters with unrelated plots separate from the main plot. Oftentimes in the Disney Doug, Doug's story is actually secondary, if he even has one. In some episodes, he is never even given a point of view - he's just the background character to someone else's story.
3. Padding and fluff: The way Disney Doug largely handled having to write every story for 22 minutes in length was by including a lot of goofy cartoony/slapstick sequences (for example, Mr. Bone literally chasing Doug and Skeeter for minutes on end when they were skipping class, to the point of following them in a helicopter). In the Nickelodeon Doug, if there was a period of action, it would either be shown as a montage, with the general gist of what had happened being shown in short clips, showing time had passed, or if it was a big moment, it would be shown in its entirety - the big, dramatic moment. On occasion, the animators made use of some "cartoony physics" to exaggerate a movement for emphasis (showing the ketchup in slow motion flying through the air in Doug Bags a Neematoad, for example), or for the purpose of humor, but then it was done with subtlety - there was no fast-paced buffoonery.
4. The writing on Disney Doug was all over the place and, in addition to not having one central plot, the plots themselves had no build up. The Nickelodeon Doug used twists, turns, setbacks, false progress, anxiety, hope, contemplation, and various other tools in a story with a narrator to tell a story and depict the protagonist's thoughts and feelings. The Disney Doug didn't tell stories, it just showed sequences of (ridiculous) events. That's really the best way to put it, I think.
5. The Nickelodeon Doug, especially in the first season, had beautiful hand-painted background and prop art with mood/ambiance. Here are two of my favorite backgrounds, one from Doug's Doodle (which is loaded with beautiful ambiance) and one from Doug On the Wild Side.
http://i986.photobucket.com/albums/ae342/dougfunnieiscrazy/Dougs%20Doo dle/37.jpg
http://i986.photobucket.com/albums/ae342/dougfunnieiscrazy/Doug%20on%2 0the%20Wild%20Side/21.jpg
This also helped tell the story, whether it was in reality or Doug's imagination, and whether it was conveying warmth, peacefulness, sadness, eeriness, fright, cold, or any other of the vast array of moods. The Disney Doug, on the other hand, had a garish color design (my guess is Disney wanted Jumbo to "standardize" Doug so that it fit in with the other Disney cartoons and their garish colors) with ugly thick, black line work outlining all the objects.
6. The voice acting plummeted - not just because of the departure of Billy West (though that was the most noticeable - Billy West WAS Doug and Roger...nobody could replace him), but because of changes in the acting choices the other cast members made (possibly the result of different directors - Jim Jinkins was reportedly not around as much during the Disney years).
7. The Disney Doug went into PSA/after school special territory with the eating disorder episode and the skipping school episode, possibly others (haven't seen enough episodes to say for sure). That was something they carefully avoided on the Nickelodeon Doug. The writers were extremely careful to avoid cliche writing and if they were going to have a message that was common, they made sure to construct it creatively and intelligently.
8. The Nickelodeon Doug was often surprisingly dark and quite cynical, while still remaining life-affirming. Doug's In the Money is a great example of the cynicism the show sometimes demonstrated. It was also often anti-authoritarian, but it still loved all of its characters. The Disney Doug had none of these features - it seemed to me that the writers, many of whom had roots in sketch comedy writing - got out of control and just made it really silly. I'm thinking Jim Jinkins did a great job of reigning them in and keeping them focused on Nickelodeon, but with a less active role on Disney, they just goofed off and lost their passion for the show and its characters and actually telling a story.