Programming wise, an RPG isn't any more complicated than other types of games. The difficulty with developing an RPG is that it requires a LOT of content, which is why it's generally good to work as a team if you plan on completing one.
It also requires a lot of different skillsets. Writing effectively would be the first obvious one. Writing is a craft that takes years of study and critique and practice and failure to develop. You also have the design elements, where you actually 'create' the world, its history, the characters (both PC and NPC, living and dead), the geography, political systems, ... You can be a world-class programmer and be completely incompetent in those areas.
Toss in art and audio development into the mix as well, stir it all up with dash of screenwriting and casting if you're using any voice overs, and you're on your way to an RPG stew. Serve chilled with Tsingtao (the only German beer brewed in China) and boston creme doughnuts.