It seems a little negligent not to share a few thoughts after a major Apple event. While Apple are, as ever, really secretive about what they're working on a few of the anticipated revelations were of course true. OS X is getting it's visual redesign and definitely shares a lot of it's aesthetic details with iOS. The window chrome is transparent, white, and layered. Everything is a little flatter and there's been a move towards iOS' vibrant iconography. There haven't been any radical alterations to the operation of the interface — after Apple's 'Back to the Mac' drive some Apple bloggers and news sites were predicting an ever-encroaching merge of iOS into OS X. I think Apple have shown that the two operating systems will remain distinct, but will co-operate more and share elements of visual style so that moving from one to the other is seamless and natural.
Speaking of co-operation, the most surprising new feature for me was the integration of iPhone functionality into OS X (and the iPad). Your Mac will essentially work as a remote loudspeaker for your phone, allowing you to answer phone calls on your Mac. Of course your phone is still doing the actual work of communicating with the mobile network; the audio streams are simply relayed across your wifi (and maybe bluetooth?) connection. Similarly text messages are relayed from your iPhone to iMessage on other devices. It's neat, very clever, and really blurs the distinction between devices — clearly the intention is to remove the hardware barrier from the things you do, all features can be engaged from the device that you're holding at the time. Nobody else can do this degree of integration at present.
iCloud Drive is another big feature. iCloud has always boxed up the files from your apps and isolated them from other apps. If you wanted to move a file between apps you had to make a copy (on iOS at least, there are workarounds in Mavericks). Now the iCloud file system will be accessible and this clunky implementation will be significantly improved.It will improve co-operation between apps in your workflows and prevent a lot of needless duplication of data. It remains to be seen exactly how well this will work, however.