To be fair, Season 3 had some good ideas. However, it was all botched due to rushing some subplots, as well as forgetting others or not giving them proper conclusion, Vlad's Villain Decay making him less impressive as a villain as well as appearing when not needed, constant Ship Tease between Danny and Sam that even shippers felt was unnecessary and unneeded episodes (one example is Livin' Large, where Danny suddenly changes his opinion on being rich in the middle of the episode and becomes greedy for no reason).
Danny finally frees Wulf, but the episode this occurs in only features this in the second half of it, the first half is more of an unrelated summer camp story.
Danielle's subplot of being unstable was going to end differently, with the Fentons adopting her, instead she's stabilized by a miracle "Fenton gadget of the day" and goes off on her own again... in only her second appearance.
Valerie learns the existence of half ghosts and that her hate for them made her a pawn of Vlad Plasmius, vowing to make him pay... right before the series finale.
Also, in the episode Valerie becomes a ghost hunter, Tucker attempts to establish a relationship with her. Although she at first dates him because of his knowledge on ghosts, the end of "Shades of Grey" implies she started to reciprocate those feelings. However, by her next appearance, not only is Tucker over Valerie, he's outright rude to her. This route could've granted Tucker some much needed character development with dueling loyalties, but was removed in favor of Valerie becoming Danny's love interest.
Several people felt "Reality Trip" would've gone better if Danny didn't erase his secret identity from his parents' memory. If they still knew, Danny wouldn't have to worry about them hunting him like in "Forever Phantom" and they would have been much more helpful if they were aware, from stabilizing Dani to knowing not to trust Vlad. If anything, erasing their memory, especially when they made it clear they loved their son no matter what, wasn't restoring the status quo so much as not letting characters develop and keep plots repetitive.
Many were also disappointed that we didn't get to see Valerie's reaction to Danny's identity being revealed.
Thanks to Steve Marmel not being a part of the show anymore, many story arcs from the first two seasons were dropped. The above-mentioned Danielle and Valerie stories were among them. Other examples being the return of Dark Danny and Vlad's deal with the Fright Knight. These could have been explored in Season 3, instead of an episode devoted to...the Box Ghost.
"The Ultimate Enemy" set up the possibility that Vlad might see the error of his ways one day and reform. Sadly, he never did.
Vlad attempting to clone Danny really didn't go past the first episode it's brought up in. If his plan had actually succeeded, we could've had some interesting plots and encounters with a clone of Danny, possibly on the level of "The Ultimate Enemy" in terms of how they could've been done.
