Some of the issues addressed include unresponsiveness when playing a video in Safari, audio not playing when using HDMI or optical audio out, Wi-Fi connection reliability, problems with data and settings migration to a new Mac running OS X Lion, and an issue with admin user accounts going missing after upgrading to OS X Lion. Details on the update can be found here.
For users with MacBook Airs and 2011 Mac Minis, there are bug fixes specific to each hardware. The update for those products can be directly downloaded here. Some of the problems resolved include an issue where the MacBook Air may boot up when a MagSafe Adapter is attached, an issue causing intermittent display flickering in the MacBook Air, and an issue that causes the SD card slot in the Mac Mini to run at a reduced speed.