When Steve Jobs was away due to his illness, Bertrand Serlet presented Mac OS 10.6 (Snow Leopard) at the WWDC June 2009 with the startling slide that said "0 new features". There was sustained applause from the crowd. But in fact there was a very significant under-the-hood new feature called "Grand Central Dispatch". It is a low-level API that removes a lot of the pain of multithreading. Essential for OS X and iOS developers, as it turns out.