While you twiddle your thumbs waiting for Diablo III to come back online after patch 1.0.4 rolls out, which is expected to happen around 1pm Pacific, why not have a gander at the full changelog? While earlier patches mostly brought small tweaks and changes, this is a big 'un aimed at overhauling large swathes of the game to increase 'enjoyment.'
The changelog has the full rundown. When you hit level 60 you'll get to grind out extra stat points from the new Paragon Levels system, while Nephalem Valor is tweaked so you get more shiny gear and bonuses. Blizzard has also made Legendary items less rubbish, fixed bugs, tarted up the auction house, rebalanced heaps of skills, and made loads more changes. Look, just read the changelog.
The North American server downtime to roll out patch 1.0.4 became at 3am today and should end at 1pm, while in Europe it'll kick off at 3am UTC on Wednesday and run until 1am.