Ear Worm: It feels so good to be bad ! So delicious to be a despicable cad ! It's just so thrilling and so fulfilling to give somebody the worst time they've ever had ! It feels so good to be bad !.
Esoteric Happy Ending: Charlie is allowed to go back to Earth and live out a considerable lifespan with Sasha and David. All well and good, except that Heaven's monotony and starched, sniffy, oppressive nature was driving Charlie to the brink of madness. He's been granted a brief, happy reprieve — then back to an eternity of misery. The only upside is that he'll live on Earth with Sasha for a few years and then the two will ascend to the afterlife together.
Harsher in Hindsight: Both Itchy and Carface ascending to the different halves of the afterlife is more poignant now that Dom De Luise and Ernest Borgnine are no longer with us. Charlie's goodbye scene with Itchy in particular has a sad tint knowing not just Charlie Sheen but all three of Charlie's voice actors throughout the franchise would outlive De Luise.
Idiot Plot: The movie would've been over in five minutes if Annabelle had sent anyone but Charlie down to Earth to get the horn. And even THAT wouldn't have been necessary if she hadn't been so trusting of Carface, who was already plotting with Red to take control of Heaven well before he steals the horn. True, she couldn't have known this for sure, but it's amazing that even those in Heaven could not clue in on what he was planning.
"During the "I Will Always Be With You" segment, Charlie and Sasha can be seen gazing towards the sunset atop what we can only assume to be the Golden Gate Bridge; notice that the actual span of the bridge for whatever reason is not there.
Most of the designs of the human characters, mainly those in the background are a noticeable step down from the original film's semi-realistic look.
There are plenty of instances of animation overlapping each other improperly if one looks hard enough, especially during the fast-paced segments.
During the chase inside the police station, the officer who got knocked out between the double doors disappears in a few frames before we see him again lying down completely dazed once more after David outwits the cops.
Much like the series, there are a lot of implied politics between heaven, hell and earth, little of which is explored. Where are the souls of the dogs who can't get into heaven when Gabriel's horn is missing? Are they in some kind of purgatory (seriously, dog purgatory sounds fascinating) or just stuck outside the literal gates?
Gabriel's horn, despite it's implied awesome powers and dangers of it falling into the wrong hands, is just a MacGuffin here and isn't shown to do much more than open things at will. What exactly is Red using it for other than Fantastic Racism? Sending them to hell? Or some other dimension of divine punishment?
The fact that Charlie doesn't so much as mention Anne Marie, let alone run into her as an older woman, considering how much of the previous movie was about him falling in (platonic) love with her.
For that matter, it barely touches upon the emotional impact coming back to life six decades after his death has on Charlie.
Uncanny Valley: Charlie and Sasha's passionate kiss that ends the movie. There's something... off about two anatomically-correct dogs kissing like humans.
Carface is now in direct cahoots with a devil-like cat creature and is shown later to be completely oblivious to the fact that he sold his soul to have his mortality restored. This comes back to bite him in the ass when Red snatches him away to answer for his incompetence.
Charlie, who trusts Carface implicitly despite being murdered by him previously.