Overall, I'm very pleased with Mountain Lion. However, I do have some quirks with it. Here is my list. Maybe some of you could help me. In Lion, you three-finger double tapped to look up in dictionairy. Now, it's three finger tap, which is also my gesture via Better Touch Tools to middleclick. Remapping look up was a pain in the ass. Safari now scrolls like iOS. That means scrolling down through a page is extremely fast after the third scroll, which messes with the rhythm I was used to. For some reason in ML I get signed out of twitter site (in safari) every single time I close the page. Mail gestures (three finger swipe up and three finger swipe down to view next/previous) don't work well. You need to scroll to the top and get that bounce effect first, and then the gestures register. There are many more very minor annoyances, but these just happen to be the most upfront.