These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Leslie Chow is either the most hilarious character or the most annoying character in the trilogy, depending on who you ask. When it comes to the sequels, you can also add people who think he's funny, but not funny enough to warrant the increased screen time in each sequel.
Alan is either the funniest character in the series, only funny in small doses or is the most annoying character of the films.
Critical Dissonance: Part II was largely panned by critics, but it was successful at the box office, outgrossing the original (in more than one way). Part III has an even more negative reception than II, but still managed to have the second highest opening weekend for an R-rated comedy film.
Funny Aneurysm: Not that it was all that hilarious before, but seeing a giraffe get killed becomes even more horrific when you find out about how they treat animals in Copenhagen Zoo. Especially giraffes.
Germans Love David Hasselhoff: The character of Leslie Chow is so popular in Japan and China that he has been given more and more screen time with each movie in the franchise.
Insane Troll Logic: The reasoning behind a Frivolous Lawsuit a month before the release of The Hangover: Part II - S. Victor Whitmill, the tattoo artist who worked on Mike Tyson's face tattoo, claims that Stu getting that same tattoo whilst drunk infringes on his copyright of the tattoo's design and the film should be banned as a result. This is despite Mike Tyson appearing in the film. While the courts found this reasoning to be quite sane, they decided to bow to public interest, and the film will indeed open as scheduled.
They Changed It, Now It Sucks: The main complaint from critics against the second sequel was that it diverged so far from the previous films that it didn't even seem to be a comedy anymore.
Moral Event Horizon: It's possible that you will think Alan drugging the Wolfpack crossed this line, although he didn't realise the drugs were Roofies and he only did it because he thought it would mean everyone had a better time. The second time around, with Alan intentionally spiking a bag of marshmallows with his ADHD medication & muscle relaxers with the intent of drugging Stu's future brother-in-law Teddy out of jealousy, appears to be a more clear-cut instance of this. However, the events of the third film convince him to turn his life around. Then The Stinger comes along, the Wolfpack (along with his new wife Cassie) is roofied yet again... and this time, he's not responsible. Guess who he received that wedding cake from?
Lisa, the receptionist at the Caesar's Palace. Helps that she's also hot.
Mike Tyson. And again in Part II
The tattoo artist in Part II was meant to be one, and be played by Mel Gibson. But some of the actors refused to work with him due to his anti-Semitic statements, so he was replaced with Liam Neeson. Then the scene needed to be reshot and Neeson wasn't available, so in the final film he's played by the utterly unrecognizable Nick Cassavettes.
Sequelitis: The first one was well-received. The sequels, not so much (the second for retreading the original while adding on Vulgar Humor, and the third for extreme Flanderization while drifting too far from the original to the point that comedy is almost an afterthought).
It's not uncommon to find people who complain that, after being a living MacGuffin in the first film, Doug is wasted as a character in the sequels by leaving him out of the drunken night in Part II & restoring him to his original role in Part III. Especially since his actor, Justin Bartha, has shown he can easily play a quirky companion in the National Treasure films.
A lot of people are also unhappy about Heather Graham's character Jade being left out of Part II, since she and Stu seemed to actually go together well in their final scene, and because she's perhaps the only sympathetic and rounded female character.
Though she does turn up again in part III.
What an Idiot: Part II was sued for copyright infringement over a tattoo. This is the thirdtime in several years this (a copyright infringement suit, this being the first suit anyone has filed on anyone else over a tattoo) has happened to Warner. They settled in June.
The Woobie: Alan is this at some points. Especially when he starts singing "We're the three best friends that anyone could have!"
Even more so when he's genuinely distressed at the idea that Doug might be dead, while the other two don't seem to have considered the possibility.
Stu certainly considered the possibility:
Our best friend Doug is probably lying facedown in a ditch right now with a methhead buttfucking his corpse!!
Alan's Woobie status drops dramatically when you consider that nearly every bad thing that happens to the Wolfpack is his fault. Mainly because he drugs them in both films. Then comes the third, where his sympathy drops further as he's becoming too weird to function.
Stu is another Woobie. In the first film, his girlfriend controls his life with an iron fist and he not only lacks the guts to stand up for himself (until the end), but actually makes excuses for her abusing and cheating on him. In the second film, his disapproving father-in-law emasculates him in a similar fashion. And to top it all off, Alan's stupidity results in Stu being put through hell in both films and nearly having his marriage destroyed in the second.