Uniquely among Slavic languages, and unusually among modern Indo-European languages, the Western South Slavic languages (Serbo-Croatian, and apparently some dialects of Slovenian) have a lexical pitch-accent system. That is, the languages permit "phonemic tone, but where only one or two syllables in a word can be phonemically marked for tone, and many words are not marked for tone at all" (Wikipedia). Modern Serbo-Croatian distinguishes rising and falling tone on syllables with long or short vowels (4 accent types).
How did this tonogenesis come about? Was it an innovation in West South Slavic? a sprachbund effect? A reflex of a pitch accent system in Proto-Slavic that was lost everywhere else?