A grande "mudança" é a nova numeração que passa de 2.6.39 para 3.0 :P
Linus esclareceu que não existirá nenhuma alteração profunda na nova versão.

"So what are the big changes?

NOTHING. Absolutely nothing. Sure, we have the usual two thirds driver
changes, and a lot of random fixes, but the point is that 3.0 is
*just* about renumbering, we are very much *not* doing a KDE-4 or a
Gnome-3 here. No breakage, no special scary new features, nothing at
all like that. We've been doing time-based releases for many years
now, this is in no way about features. If you want an excuse for the
renumbering, you really should look at the time-based one ("20 years")
instead."

Veja aqui a mensagem de Linus: https://lkml.org/lkml/2011/5/29/204

Resumindo, a versão 3.0 servirá mais como um "apontamento histórico" do inicio da terceira década de existência do Linux.