I've seen The Hobbit about four times now, and I squealed out loud in the theater when the dwarves sang and the Wargs showed up.
So please, nobody come after me with an axe when I tell you that The Hobbit is a terrible adaptation.
Is it a terrible *movie*? Well... I like it, and not in an ironic or snarky way. I genuinely enjoy the movie. But does that make it good? Not really.
See, the problem with The Hobbit is that all of its good parts come directly from the book that it adapts. Which makes me think that it would've been a great movie IF THEY'D JUST STOPPED PADDING THE DAMN THING.
The Lord of the Rings was a cultural phenomenon that changed the way movie franchises were created, sold, and marketed. You could argue that Harry Potter also helped, but I think the main way that Harry Potter contributed was as backup, to confirm that LOTR wasn't a fluke. Because the Harry Potter movies suck (sorry people who like them, they do!). LOTR is a series of skillful, masterful movies that are both fun and emotionally effective. It's hard to argue with their merit.
The Hobbit is their successor in a whole lot of bad ways.
Right now we're living in the age of the superhero. We've mined so much material from comic books that we've actually created a hydra of Marvel movies that operate together as well as separately (at least you brought me Jeremy Renner, Avengers).
Superhero movies are a testament to brand name recognition and the hesitance of investors in uncertain economic times. New scripts are unknowns, you have to take a gamble that they'll catch audience eyes. But known franchises? Hey, you've got a built in audience! Why would you pass that up?
And so we come to The Hobbit, which has been stretched out of a relatively simple, jovial children's book into the bastard son of LOTR, the Silmarilion, and studio executives hissing, "MOOOOOOAAAAAARRRRR."
Here's the plot of The Hobbit: Dwarves want gold, so Gandalf helps them put together a team and they traipse across Middle Earth to find it.
Here's the plot of the first Hobbit movie: Dwarves want gold, so Gandalf helps them put together a team, but Thorin's whole family fell to gold-lust and got involved in a blood feud with this one bigass orc who comes after them right now because shut up he was busy for the last 50 years, but also The Dark Is Rising in the forest of this wizard so he's coming after Gandalf to get help with the Necromancer and *pant pant pant*
The best parts of The Hobbit movie are taken straight out of the book, and that's not just the nostalgia goggles talking. The problem with adding in Bigass Revenge Orc and Necromancer and Totally Not Evil Saruman is that these things distract from the goal of the journey, which is to reclaim the homeland of the dwarves. In LOTR, the goal is to kill the ring. We split our focus and follow the plight of the peoples of Middle Earth in order to understand WHY this ring has to be destroyed. The split focus is necessary in order to give Frodo's quest weight.
In The Hobbit, why is our focus divided? Well, you could say that Bigass Revenge Orc helps develop the characterization of Thorin. But why? Thorin is the leader. He wants to get his home back because he's king. We don't need another reason. We understand the quest for a rightful throne.
And as for the Necromancer, Radagast, and the Not Evil Council, they affect the company of Thorin and Co. not at all. Their sole result is to act as a prequel to films that have already been released. They serve no narrative purpose whatsoever.
Peter Jackson split The Hobbit into three films for the sole purpose of making money, and possibly to add some bonus geekery, and the film is infinitely weaker for it. There was no narrative purpose to the changes they made. So yes, The Hobbit is a bad adaptation, because it is not an adaptation of the The Hobbit! It's an adaptation of The Hobbit + The Silmarilion + Some Shit We Added About Dwarf Wars and Single Revenge Orcs + Studio Greed.
The Seeker is the DOA nominal adaptation of Susan Cooper's famous The Dark Is Rising young adult fantasy novels. The screenwriter they hired "didn't like fantasy". The story was immediately changed both for reasons (to make it more like Harry Potter) and for ...reasons (she mentions Vikings! Let's add in a ton of Vikings! The kids love Vikings, right?). The main characters were changed from English to American (even though the movie takes place in an English village), the plot was torn apart for no clear reason, and even the name was changed.
Odds are pretty good you've never heard of it, and there's a reason for that. The author hated it, the movie bombed immediately upon release, and it proudly holds a 14% approval rating at Rotten Tomatoes.
As much as it pains me to use it as a (slightly) more positive example, while Transformers is also a bad adaptation, Transformers is aided by the fact that there had already been several different incarnations of the source material. It operated in a state of flux, so nobody was too surprised when Michael Bay took what he wanted and threw out the rest. But I ask you this: why did Michael Bay make a Transformers movie and turn it into a PG-13 CGI actionfest? Was it because he loved the source material? Hell no. It was to cash in on that sweet, sweet brand name recognition.
Probably some people can change things purely for Teh Profit and reap the rewards ( in fact, Jim Butcher stated about his hugely successful Dresden Files series that, "When I finally got tired of arguing with her and decided to write a novel as if I was some kind of formulaic, genre writing drone, just to prove to her how awful it would be, I wrote the first book of the Dresden Files."). But generally speaking, I think it's almost a guarantee that if you change something solely because you want to cash in on a trend or stretch out the material, it's going to fail, or at the very least be unrecognizable as an adaptation.