Also has a massive tendency to repeat jokes. One example being the "three legged cat" joke.
Dan made Drake & Josh an entire series about main characters being pranked by a young kid, and he's made half a dozen episodes of iCarly on the same premise.
He copies a lot of short one-line jokes or physical comedy bits from Friends, and before you think it's a coincidence, Dan loves Friends and knows it like the back of his hand, as well as knowing David Schwimmer:
The entire Spencer is dead plot is completely stolen from a plot involving Ross.
Sam teaching Spencer how to lie, ripped off from Phoebe teaching Joey how to lie.
A physical joke where Spencer is sad and lies down on the kitchen table, stolen.
Melanie the twin plot. Identical to a Phoebe plot-line.
Chandler remarks about how travel sized items make him feel like a giant. Freddie says the exact same line.
Constantly posting still pictures or movies from the set implying romantic developments. He's done this with Sam/Freddie & Carly/Freddie on iCarly, Beck/Tori, Cat/Robbie, Jade/Beck andJade/Tori from Victorious.
He has teased Sam/Freddie & Cat/Robbie shippers while making Sam & Cat, with the idea of the male parts of those pairings showing up on the new show. And, of course, he's turning up the teasing on Sam and Catthemselves.
His blatant revisionism via his post-episode 'fun facts', where he gives loads of attention in the blog posts to the pairing that wasn't just explored. Downplaying the romantic events in the episode or trying to reverse and muddy the waters constantly.
Throw It In: If a blooper is funny enough, he'll throw it in the final cut of the show.
What Could Have Been: Dan has implied that the Disney Channel stole an idea he pitched years earlier for a show that eventually became Sonny With A Chance. Had Disney picked up his show it's quite possible he'd have never worked for Nick again. Depending on exactly when he pitched it could have meant that Zoey101, Drake & Josh, iCarly, Victorious as well as Sam & Cat either never got made or got made in Disney form. Considering those shows formed the core of Nick's live action for the better part of a decade it would have changed things immensely. As one random example, if Drake & Josh ended up on Disney, it might have been Miranda Cosgrove who became Hannah Montana and not Miley Cyrus.
Then again Dan has stated in his own blog that there's "nothing preventing him" from working with Disney and implied that it could happen in the future for whatever reason (the link is even in the main article's Tropes page). This implies that at the very least he feels no bad blood over the situation if it was indeed true. At the very least it somewhat clouds the issue.