I have no idea what all of the behind the scenes APIs and system enhancements are, but the improved security is welcome. So this list will focus on user interaction. Pros • Quick Look o Right sizing o Before when you looked at something small, hitting the down/up arrow made the next file the same size even if it should have been larger. And vice versa, something small would be stretched to make it bigger. o Now hitting the down/up arrow makes everything look the right size. o I wanted that all along in Snow Leopard.• Mail o Email threads look like separate pages. This makes following all of the threads easier. o The overall GUI looks better. o But I don't get the "See more from…" feature. It doesn’t function how I would have expected it to, and how do you un-see them? o The iOS like side bar is ok. I don't love or hate it.• Resizing Windows o It’s nice to be able to do this from all sides, although track pad precision isn't all that great for resizing.• Hiding scroll bars is nice. • Finder o Air Drop is slick. Its UI could look better though. o All My Files is interesting, but how functional is it if you have a LOT of files?• Mission Control o Swiping left and right to switch spaces is nifty.• Hot corners o This is nothing new, but setting hot corners for Mission Control, Launch Pad, etc is an essential feature.• Rubber band effect o I don’t love it or hate it.• Launch Pad o It’s not that bad. I’ll probably use it for things not on my dock. Cons • Finder o Lack of pinch to zoom of icons in Finder, like icons of photos. o Nooooooooo. Why, why, why was this taken away?• Full Screen o The dock isn’t accessible when in a full screen window, and it’s not really acceptable to switch to another space to use the dock. o Safari isn’t technically full screen if you can’t hide the address bar. Accessing it via mouse over would work. o Was shift+cmd+F taken? opt+cmd+F is awkward. o Why is a full screen window in a one to one relationship to a space? • Mission Control o Again, why is a full screen window in a one to one relationship to a space? o Coming from someone who used Spaces a lot (and liked my spaces in the space I put them) but never used Expose or Dashboard, Mission Control is a cluttered mess. o Using it was jerky, and that was on a new, pimped out Mac Book Pro. o Can't move a window to another space using hot corners. o Moving windows from one space to another in the old way was simply better. o Opening new window of an application already in a different space was something I always wanted, but the way it works, I’m not sure that it works how I would have liked.• Versions & Auto Save o Ok, I can see this being a life saver. Does anyone remember getting the bomb in Classic OS at 3:00 AM writing a term paper and breaking into tears? And I can see myself getting very used to not saving things, but it could have been implemented better. o The big deal breaker about it is if I make significant changes to a file (and this is often) I no longer consider it just another version, but it’s own file. The inability to “save as” a new name at that point is unworkable. o The Time Machine like versions interface could also be easier to use, such as forward and backward gestures to view through the versions and hitting return (or cmd+return) to select that version. Also who wants dozens upon dozens of versions, so cmd+delete would be nice too.• Resume o It’s good in theory, but it’s a bear to reload all of the apps/windows/tabs. Again, this was on a new, high end MBP.• Gestures o Ok, I haven’t explored them all, and they aren’t all bad, but I have to wonder about when too many is too many. o Some are intuitive and an excellent way to interact with the OS and applications, but others are too arcane and awkward.• iCal and Address Book o You’re kidding me, right? Ok, this was a thoughtful assessment, so thoughtful replies, comments, and corrections would be more appreciated than down voting.