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.
Ferris is a sociopathic Manipulative Bastard who only cares about toying with others. The Spoony Experiment has fun with this one. As far as sociopathy goes, if you look at the The Sociopath analysis page, you'll notice that Ferris ticks a lot of boxes; superficially charming, extremely manipulative (for example hitting Cameron and then managing to guilt-trip him immediately afterwards), no concern at all with making other people's lives miserable (with the one exception of when he offers to take the heat when Cameron kills the car), no long term plans, totally impulsive and with no real appreciation for the consequences of his actions, trusts his own ability to make up lie after lie on the spot, contemptuous of other people's emotional states (notice his mockery of Cameron's deep-rooted fears and insecurities), and truly struggling to understand the way normal people's minds can work (look at his genuine confusion when he suggests Sloane marry him and she, quite rightly, shoots him down).
Conversely, another popular theory is that Cameron is the hallucination in Ferris's head. This allows the plot to remain mostly intact while adding insight on him. This is why Ferris is so confident, assured,and suave (i.e. unlike the vast majority of teenagers); he takes all his fears, doubts, and insecurities and shoves into this alternate personality he calls Cameron. This also explains why everything works out and he's universally revered; it's Through the Eyes of Madness though not to a severe degree. This is why Cameron's tenderness with Sloane is allowed, because Ferris wants to be honest with her and show her this other, more real side of himself.
Likewise, Jeannie is The Chew Toy. She tells the Cassandra Truth that Ferris isn't sick and is called heartless by fellow students. She tries to prove that Ferris snuck out of school (and in a manner not nearly as outrageous as the principal's) and ends up arrested and grounded by her parents. By that point she just goes with it, and to many modern viewers (particularly girls and people who understand rules) she's just been through so much that she's clearly given up. The only thing she did wrong was cut school, which Ferris also did in addition to many, many other wrong things, and yet she was the only one punished for anything, including stuff that she didn't do.
Quite a few modern teens have even said they wanted her to "win" and turn in Ferris to be punished. She managed to not come off as a bad person like Rooney (or Ferris) and her giving in at the end (supposedly a sort of Heel-Face Turn) was seen as someone standing up for what they believed in being forced to cave to someone more popular. It also might have had something to do with finding Rooney's wallet and figuring she's ok with Ferris going unpunished if it means Rooney gets his for scaring her into thinking there was a rapist in her house.
One Alternate Character Explanation has Ferris as a kid who's good at heart and genuinely wants to help Cameron and other classmates, but goes about it the way he does because he's, well, a teenager. Sort of like a Knight Templar, but not evil, just egotistical. And really lucky.
Or maybe Ferris has sociopatic tendencies he does-as stated above-shown a lot of signs of sociopathy, but the movie also shows he does generally care for Cameron as he was willing to take the fall for Cameron which is something that a real sociopath is unlikely to do.
Big Lipped Alligator Moment: The "Twist and Shout" musical number, where seemingly everybody in the city of Chicago starts dancing along.
Canon Sue: Ferris gets away with everything and EVERYONE loves him to a ridiculous degree, (although it's slightly justified because he also gets other students out of trouble) except for the villain characters who are treated unsympathetically. He shares many traits with a Black Hole Sue, as the universe seems to revolve around him in a decidedly unrealistic (though entertaining) way.
"...Everything works for him. There's nothing he can't handle. I can't handle anything. School, parents, future; Ferris can do anything."
Interestingly enough, John Hughes said in the director commentary that the reason he picked this song as the theme song is because he thought it was awful, and completely emblematic of terrible 80s music.
Designated Hero: Alternate Character Interpretation holds that Ferris is spoiled, immature, selfish, obnoxious, and manipulates close friends and family for no reason beyond his own amusement. Despite his near lack of redeeming qualities, his name's in the title, so you're supposed to root for him.
His offer to take the heat for Cameron (which potentially could mean worse for Ferris than it would for Cameron) at the end shows that he does have redeeming qualities though and he even admits in an aside that he genuinely wanted to show his friends a good time.
Alternately, regardless of any real or imaginary traits in Ferris's moral character, he is still the designated hero simply because, if you think about it, Cameron is the real protagonist.
Designated Villain / Informed Wrongness: Rooney is depicted as a Dean Bitterman-type who's seemingly trying to stop Ferris and his friends from having fun for no good reason. Except he does have good reason: it's his job to enforce school regulations, and Ferris has been breaking said regs by skipping school at least nine times before he hacks into the school computer to alter the records, and does so by blatantly exploiting the good will of everyone around him, including his parents. Yet, the movie turns the audience against him by having him go way too far in trying to catch Ferris; breaking into his house and assaulting his dog and having him act as though he's trying to catch Ferris out of spite instead of trying to enforce the rules.
As this Cracked Article points out, due to Ferris' absence being excused there's no legal precedent for Rooney trying to bring him in, and even if he did expose what Ferris was up to, it wouldn't have led to anything. Rooney is doing what he's doing because Ferris has been a bad influence on the school and even hacked the school computer and he wants to get back at him, even if he doesn’t have much of a legal leg to stand on.
Jean's character arc comes down to letting Ferris get away with his behavior by accepting who he is and moving on with her life, because all of her attempts to expose him have only given her more grief than Ferris ever did to her personally. It's a stark contrast to other aesops that make you think you are only validated if you can achieve an unreachable goal.
Druggie: You oughta spend a little more time worryin' 'bout yourself, and a little less time worryin' 'bout what your brother does...
Ron the Death Eater: Skipping school to have fun with his friends? Ferris Bueller is a monster!
Strawman Has a Point: Rooney's quest to prove that Bueller is ditching school is a legitimate concern. Most schools in America are legally required to maintain student attendance, or they face the possibility of having their funding cut.
Suspiciously Similar Song: The Ferris Bueller TV series employed a little musical riff that seems awfully similar to Yello's "Oh Yeah". (F-F-F-Ferris B-B-B-Bueller hee hoo!)
If mobile phones were as widespread then as they are today, it'd be harder for Ferris to bluff his parents if he could be contacted anytime, anywhere.
It would have been a lot harder for Ferris to pretend to be Abe Frohman, the Sausage King of Chicago, since the real Abe Frohman would most likely have a webpage either for himself or for his company today complete with at least one photograph of himself.
A video of Ferris dancing on the parade float would have most likely made its way to Youtube today, and the jig would be up.
They Just Didn't Care: The aforementioned TV series, bordering on In Name Only. Virtually everything that made the film so memorable was stripped away — the setting was changed from Chicago to L.A., the soundtrack consisted of a near-endless loop of Casio riffs fit to make one's ears bleed, Ferris was now an obnoxious nerd who only believed he was wildly popular (and attended school without complaint!), Cameron was still neurotic but lacked the tragic Backstory from the film, thus making him a figure of fun, Sloane was now catty and shallow note and they couldn't even be bothered to spell her name right — on the sitcom, she's Sloan Peterson, Ferris's parents were now named Bill and Barbara instead of Tom and Katie, and the list goes on and on. Not surprisingly, it lasted a grand total of 13 episodes. Not even a very young Jennifer Aniston (as Jeannie) could save it.
Jerkass Woobie: Jeannie have her moments too, though its more of this trope.
Not too much though. A lot of modern teens (especially ones who understand rules) saw her as more The Chew Toy. She could either A) right a wrong (her brother manipulating her parents and the whole school, which he had done before—9 times) and stand up for what she saw as right and wrong or B) give up. Her giving up was supposed to be a good thing.