I think that some people just take a long time to release a record. Sometimes is perfectionism (My Bloody Valentine), sometimes they have the music but just don't care enough to release to the audience (Lauryn Hill), sometimes they just don't have what to write about (Fiona Apple), sometimes they go through a hard time personally (Amy Winehouse), sometimes the album was released in 2006 but 3 years later it still sounds like it was released yesterday because of staying power and radio presence (Winehouse, again), sometimes they "retire" (Joni Mitchell) and sometimes they have kids to take care off and just don't write a thing (Kate Bush). I guess some artists are too aware of pressure and how big they are. When you can't deal with high expectations well, things like this tend to happen - not only in music. The thing about not knowing how to deal with this is that expectations only go higher with time. I don't know why Dre is nervous because 2001 wasn't a masterpiece and judging by the material released: the album is what hip-hop classics are made of. If he's being a perfectionist, he's doing it all wrong.

Maybe he just can't find a day where all 57 featured rappers are free?

Or he's running out of Motown singles to change slightly, rap over and hope his fans are young enough that they haven't heard them.