"The movie is long."
A long discussion on this at another sentence thread: https://www.duolingo.com/comment/24450404
Cross-linguistically, /l/ is a weak sound and prone to changing to a vowel or even disappearing entirely. This happened in Mandarin when /l/ was (at some point) in the syllable coda. Sino-Korean words preserve the /l/, but Mandarin lost it later on. In Korean, the /l/ is prone to vocalizing at the end of verb and adjective roots when followed by a consonant-initial affix like ㅂ니다, possibly to /ɯ/ first which then is absorbed by the preceding vowel causing the disappearance of the /l/.
It's a rule when conjugating to formal polite form. With a verb that ends ㄹ, you drop the ㄹ and add ㅂ니다. http://www.sayjack.com/blog/2010/06/17/formal-polite-form-of-korean-verbs-and-adjectives/