Which interface you mean? The one presented to a "regular" user or the one to achieve more "advanced" stuff? Remember N900 is primarily a consumer product, despite the fact geeks can have a ball, too. It's a fine balancing act Nokia had to perform here, and I think they did it quite well, even if they made things more geek friendly.

PS
Full disclosure: having mentioned Nokia, I must say I work for them, too. However, I am in no way associated with Maemo or N900 projects. My opinions expressed here are my own and do not necessarily, or even at all, reflect official Nokia position.