The eponymous hero of The Flash (1990) was an amalgamation of the Silver Age and Post-CrisisFlashes in the comics. While his secret identity was that of Silver Age Flash, Barry Allen, some aspects of the character (like his relationship with scientist Dr. Tina McGee and his need to eat insane amounts of food to maintain his powers) were incorporated from the character of the later Flash, Wally West.
The miniseries version of Hogfather and The Colour of Magic managed to retain most of the good material from the original novel, though it was apparently hard to follow for those who hadn't read the books, as it assumed you already knew most of the backstory. In this case, the distillation is probably because Terry Prachett was heavily involved in both productions, even having cameo appearances in the last scenes of Hogfather and the first of Colour.
One specific example: They based the Patrician on his later appearances (including Wuffles), instead of his eventually rather contradictory appearance in the actual early books. The "Machiavellian Vampire Flamingo" Vetinari was introduced approximately at the same time as the name "Vetinari".
The J-Drama form of Boys over Flowers managed to compress thirty-six volumes of manga written over a period of eleven years into a much smoother story, combining characters and editing plot arcs as necessary.
The Nobuta Wo Produce J-drama was based on a book whose title character was an overweight, unattractive boy, and the main character was a cold-hearted Jerkass who only wanted to produce Nobuta because he was bored. In the drama, Nobuta was a lovable Woobie girl who wasn't even capable of smiling properly, Shuji was misguided and selfish rather than a cold jerk, and the character of Akira was introduced. The resulting drama had an ending that was not saddening as the book, had beautiful cinematography, and mind-blowing plot and characterization.
The Incredible Hulk: A very loose adaptation of the Marvel Comicscharacter. The comics' supporting characters and villains are left out and only once during the series did the Hulk battle another superhuman character. Also, for the majority of the series, the only sci-fi or fantasy elements were the Hulk himself. With the exception of two TV movies, the rest of the Marvel Universe wasn't even referenced and the name Hulk was rarely used onscreen to refer to Banner's alter ego. The format for the show was a loose adaptation of Les Misérables with David Banner as Jean Valjean and Jack McGee as Inspector Javert. Comparisons to the Kung Fu TV series are also common with Banner as Kwai Chang Caine as is Richard Kimble of The Fugitive. The show focused on character drama instead of deliberate superhero-style adventure.
The Vampire Diaries: Bonnie in the TV series is a combination of the book version of Bonnie and Meredith; Meredith's traits are folded into Bonnie and bitchy Caroline takes Meredith's place in the trio.
The Hardy Boys/Nancy Drew Mysteries cuts most of The Hardy Boys book series' supporting cast. The Hardys' mother, Laura, is dead, and the boys live with their widowed father and Aunt Gertrude, and the only friends from the books that show up are Callie Shaw and Chet Morton and Chet, only in two episodes. In second season, the series is distilled even more, with even Aunt Gertrude and Callie getting cut.
Oobi stars bare hand puppets who speak in simplified sentences. It's known for and often defined by its simplicity, but the Iranian adaptation of it (Dasdasi) managed to be even simpler by getting rid of every supporting character and focusing solely on the main family of puppets.
Smallville takes several cues from the Silver Age (friendship with Lex, Clark having a sort of heroic career while in high school, supporting cast getting powers every other week) as well as Post-Crisis (Clark playing football, Clark getting his powers on the on-set of puberty) and the films (Several Mythology Gags).
Les Revenants (Rebound as titled in English) is based on a 2004 zombie movie that had a lot more people resurrecting. In the series, there are only five "Revenants", mostly to get a better assessment of their predicament. The movie also didn't provide any explanation for these unexpected resurrections, which the series plans to do eventually.
The 1981 television adaptation of The Day of the Triffids crammed a whole novel into six 50-minute episodes by the simple expedient of cutting all the Padding, and was frankly the better for it.
The Magicians: In the first book of the trilogy that the show is based on, the entrance exam had a long section devoted to it. Here it's compressed to just showing us that the writing on the exam paper frequently alters, and Quentin passes while Julia doesn't.
Wonder Woman (1975) simplified the comics. None of Wonder Woman's supervillains ever appeared, for example, though some of her Nazi opponents did. Etta Candy was demoted from leading literal charges - and winning - against various bad guys to complaining about photographers stiffing her for $1.50.
The made-for-TV Hercules condenses his early life and heroic feats by reducing his famous Twelve Labours to just six, combines several characters associated with his legend such as his mother Alcemene, who is combined with Hera and becomes a Evil Matriarch devoted to destroy him.
The White Queen: Three novels (specifically The White Queen, The Red Queen and The Kingmaker's Daughter) and 21 years of events are squeezed into a 10-episode miniseries.