For me, it's stuff like my quad core iMac becoming unusable under heavy disk activity, or issues like scrolling web pages being impossible when running (quasi)

For me, it's stuff like my quad core iMac becoming unusable under heavy disk activity, or issues like scrolling web pages being impossible when running (quasi) full screen video in a dual monitor setup, or mouse lag under ML. Minor irritations like reworked Spaces and full screen mode making a second monitor pointless are irritations, but I wouldn't regard them as reflective of the quality of Lion and ML.