How is DU not an RPG if it greatly focuses on actual roleplaying of your character? Its even mentioned in the section about the Moon Secure Areas. People need a safe space to roleplay, it doesn't only mean having character progression. DU is mainly an RPG with elements of world building, thats for sure. For a pen&paper RPG enthusiast as me this was the selling point of the game. I am free to create a group of players, roleplaying our characters and interacting with other groups without breaking our immersion. You don't have "roleplaying wars" as in theme park MMOs, you can declare a war on someone and actually care it out through in-game mechanics, all while immersing yourself in being the character you play.
All in all, DU gives you a lot of means to roleplay your character. And about the hero focus, I think you meant "protagonist". In RPGs you don't always play as a hero, look at Kingdom Come - a son of a smith that just happend to be in the middle of a war. In DU our characters are our protagonist, the difference is that you get to name him and choose his or her destiny. That is why I have high hopes of DU becoming a great MMORPG.
The point is that for you the perspective of roleplaying is limited to a predetermined character given in a game, thats why you are so hesistant to call it an RPG. DU and many other RPG games require you to create and shape your character from scratch, as in any tabletop RPG game. There are RPG servers in ArmA, even tho you have no main story and a "hero" there. San Andreas, which I don't really like, is also a good example of RPG games without your standard progression and a protagonist. People were and are roleplaying faction politics for a long time now, I think this is my final proof.
You are right about world building, its a great part but don't forget that its carried out by roleplaying people. Everyone has to roleplay in a sandbox like this, thats the point, even if they roleplay just a little.
Ye, DU is a juicy RPG and I love the premise, I hope it will be carried out well.