There are some that are nice stories in their own right, yet couldn't really be dragged out to feature length. The Gingerbread Man, Goldilocks and the Three Bears, The Elves and the Shoemaker, The Three Billy Goats Gruff and The Emperor's New Clothes are all examples, as are many Aesop's Fables.
There are also many stories which are so gory (and obviously unintended for children), that to sanitise them would leave them as extremely loose adaptations. Bluebeard (featuring a man who butchers his wives and leaves them for dead in a secret tower) and Ali-Baba and the Forty Thieves (murder is throughout, and the villains are killed by having hot oil poured over them) are the main culprits, yet there's also Donkeyskin, a fairy-tale by Charles Perrault that is still popular in France, yet probably not so in the UK/USA for obvious reasons. Whilst the idea of an oppressed runaway princess having her riches restored when the Prince of the land she's run away to falls for her may sound nice, I don't know how they'd get around the fact that the Princess ran away because her father had the hots for her.
I also feel that there are some stories which could be really good material, yet might get confused with films from other studios based upon the same works, so execs might not want to pursue them. The Wizard of Oz is the main one here, and I think that Thumbelina, The Nutcracker and Puss in Boots (due to Shrek) could also meet a similar fate.



