While I agree -- plenty of TV-to-film adaptations are hamstrung by keeping too close to a premise that's either out-of-date, too silly or just not right for a feature -- an adaptation does also need to have a fundamental understanding (and fondness) for its source material. The "21 Jump Street" movie certainly has that in its idea that youth culture shifts so fast these days that a whole microgeneration has passed since its main characters were in school -- a problem the pair's small screen predecessors didn't have to worry about then, but certainly would now. Just as troublesome as an adaptation that's worshipful of a series that never warranted it is an adaptation that has no grasp of the appeal of that series in the first place.
In resurrecting a show concept for the big screen (and for the purposes of this list, I'm setting aside films that are direct continuations of series like "Serenity" and "Sex in the City"), a surprising amount of would-be franchises have felt the need to dismantle fundamental aspects of the series on which they're based. Maybe the idea is to stake out fresh territory, but it also seems calculated to annoy fans of the originals by reminding them how little the folks behind the adaptation actual care. Four examples that come to mind, all from the '90s:
Adapted from: "The Avengers" (1961-1969)
The beef: John Steed and Emma Peel get together
The Diana Rigg-as-Emma Peel era of the surreal 1960s British spy series was in many ways its high point, and was definied by the ambiguous relationship between Peel and Patrick Macnee's John Steed. Peel was married, though her husband had gone missing in the Amazon and was presumed dead, and she and Steed had a bantering, flirty and also strikingly equitable partnership in which there was obvious chemistry and trust. But did they ever do the deed? Rigg and Macnee disagreed when speculating on that point, and there was never definitive evidence up to the moment when Peel's husband returns and the character departs the show in a touching goodbye in which she's referred to, for once, by her first name. Which is why it's so irritating that in the terrible 1998 big screen adaptation Uma Thurman and Ralph Fiennes' version of the characters get a big smooch out in the snow. The TV Peel and Steed never got that kind of closure, they were all about the bittersweet, unspoken connection.
Adapted from: "Mission: Impossible" (1966–1973)
The beef: The mole is inside the IMF
Brian De Palma's "Mission: Impossible" launched a film franchise and contained some iconic action sequences (consider Tom Cruise catching a drop of his own sweat in order to not set off alarms when breaking into Langley), but its treatment of the original series was garbled and did some pointless reworking of a major character. [Spoilers for the movie to follow] The problem wasn't even so much that IMF director Jim Phelps (played by Jon Voight in the adaptation) was turned into the main antagonist (to the displeasure of original star Peter Graves), it was that the film got no boost from this fact -- other than to make an almost spiteful display of killing off one of the primary characters of the series, there was no reason for Voight's villain to share that name.
Adapted from: "The Mod Squad" (1968–1973)
The beef: Captain Greer gets killed
Like "Mission: Impossible," the 1999 movie adaptation of "The Mod Squad" established the premise on which its TV show source material was based, then imploded it. You have Claire Danes, Giovanni Ribisi and Omar Epps taking the places of Peggy Lipton, Michael Cole and Clarence Williams III as hip, young deliquents recruited into becoming hip, young law enforcement agents. And then you have Dennis Farina as Captain Adam Greer (the role originated by Tige Andrews), the father figure who saves the three from jail to start the squad despite his colleagues' low opinion of it, and who gets... killed midway through the movie, leaving the troubled trio to investigate the drug ring/dirty cop scenario that got him offed by themselves. Given the original show was a kind of proto-"21 Jump Street" in which the kids went undercover to infiltrate various counterculture situations, the movie's doing away with the only tie to the legit side of things the three have and turning the story into a delinquents-gone-good versus policemen-gone-bad feels both pandering and missing the point.
Adapted from: "Lost in Space" (1965–1968)
The beef: Time travel
The original "Lost in Space" began as a sci-fi take on "The Swiss Family Robinson" in which a professor, his wife, their three kids, a military dude, a lovable villain and a robot ended up, as promised, lost in space. It was distinguished by a certain fundamental sameness that went beyond the usual TV stability -- the Robinsons went through wild adventures thanks to being stranded but were generally unchanged, and despite all of the trouble the cowardly, self-serving saboteur Dr. Zachary Smith got them into, they still kept him around. In adapting the series, it made sense that the 1998 film ditched the campy quaintness of the show for something more serious and effect-friendly, but they also did away with much of the "lost" scenario in favor of a convoluted time travel plot involving alternate versions of the characters that were fundamentally embittered or evil. The idea is no longer that they're a family (plus a few) holding together against and exploring a strange new galaxy, instead it's that all of time and space has been warped just to reinforce family values. In space, no one can hear your daddy issues.