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.
Complete Monster: Most of the series's villains are difficult to take seriously. That can't be said of Mister Mind, a two-inch long parasitic worm from Venus. As his people's advance scout on Earth, Mister Mind took control of Captain Marvel's cousin, Sinclair Batson, mutating him into a monster. When Captain Marvel and Mary Marvel foiled Mind's plans, the worm vowed revenge, and bodyjacking Sarge Steel of the Department of Metahuman Affairs, set in motion a plan for a nuclear holocaust. Incubating his larvae in sores within Sinclair's body, Mind took mental control of his offspring after they were born and had them in turn infest nuclear technicians, army officers, and politicians, while he himself used Sarge Steel's authority to obliterate Fairfield, Captain Marvel and Mary Marvel's hometown. Defeated before he could end the human race, Mind broke out again during the events of 52, and after mutating into his adult form of The Hyperfly, tried to devour the totality of space/time, plotting to end the multiverse. Motivated by nothing more than anger at Captain Marvel for embarassing him, Mind is as bad as The Big Red Cheese's Rogues Gallery can get.
Many fans consider the new "Shazam" to be this as well. Stripped of his previous idealism, sincerity, good humor and optimism, Billy comes off to many as a rude bundle of angst. Images◊ like this◊ didn't help matters, as he appears like something right out of 90's image.
Billy's foster sister Darla in the New 52 stories was quite popular over on Scans Daily, whose readers hoped that she would be given Shazam's powers as well. At the end, it was shown that Billy could share his Marvel powers with all of those he considered his family, but could only share it with so many before the magic fizzles out and they return to normal.
Despite many fans complaints about the treatment of Billy and Freddy in the Nu-52, the new take on Mary has been considered to be very refreshing, allowing her to keep much of her old 'nice girl' personality while also giving her a lot more backbone than other books did.
Narm Charm: Fueled by this. If you think a man shouting "SHAZAM" at the top of his lungs fighting a talking, two-foot caterpillar with mind powers can't be awesome, you're wrong.
The Scrappy: Osiris was deliberately built up as this in 52 to build up to an Alas, Poor Scrappy moment. Fans were still annoyed when the series' Blackest Night tie-in, which was billed as being about Billy and Mary having to fend off a zombified Osiris without their powers, wound up being about Osiris bumbling around confused. Things were not eased when Osiris was used in Eric Wallace's run on Titans, where he basically devolved into the Marvel Family's version of Superboy-Prime.
Ethnic Scrappy: The early character of Steamboat, a bungling, subservient black "pickaninny"-type character with enormous red lips and a stereotypical pidgin dialect, was so offensive, even by the standards of the 1940's, that he was quickly dropped and never spoken of again after the Youth Builders, a multi-racial student organization based in New York and Philadelphia, began a letter-writing campaign to Fawcett. C.C. Beck later tried (very unconvincingly) to defend the notion behind his conception - namely, that it was a misunderstood attempt to reach out to a black audience - but even publisher Will Lieberson mentioned afterward that he didn't like Steamboat in the first place.
Seinfeld Is Unfunny: Many trends that are commonplace in comics today, like a female counterpart and continuity between issues (including continuous stories) were popularized by Captain Marvel way back when.
Captain Marvel was the first comic to feature the hero's villains teaming up to fight him.
Shipping: Many fans want to see Billy hook back up with Stargirl, even badgering the new writers of the book about it. It ain't happening, since they're now handled by two different editorial teams.
They Changed It, Now It Sucks: The New 52 version of the Marvel family is nothing but this trope. Whether it's Captain Marvel's name being changed or Billy Batson's personality being changed to an edgy emo kid, almost every new development causes fans to hate it even more.
Ugly Cute: The version of Mister Mind that appeared in The Black Ring: he looks like he escaped from an anime. Or a stuffed animal collection.