A few quick OSX 10.9 feature suggestions
1) Unlock machine locally via PIN instead of password. If you're like me and use a long/complex password it's more than a little annoying to constantly type it in. For any local tasks like unlock or software installs I think a short PIN code would be good enough. Make it optional of course and limit where PIN authentication can replace password authentication.
2) Quick settings iOS style by using a two finger swipe on the Dock. Include volume, AirPlay, universal playback controls, etc. You can access most of these features via the menubar today but I think we're all suffering from some menubar inflation these days. It's getting harder to just quickly pick the 2-3 most commonly used settings up there.
3) Demote Dashboard back to the old Tiger style overlay mode and make your left swipe access a hybrid LaunchPad/Spotlight feature instead.
4) More iCloud settings syncing everywhere. Just about every bundled app should support this. For example saving my custom color profiles/settings for Terminal to iCloud would make switching between my two Macs way more pleasant. This should also be extended to things like account setup in various apps. For example Messages on two Macs should be able to share the same settings via iCloud.
5) Create a camera roll feature for OSX. Basically this is already how the iPhoto item in open dialog boxes works but again for consistency I think it wouldn't hurt to replace this with a separate camera roll feature that is integrated with iPhoto but works like the iOS camera roll. If you happen to have a big iPhoto library you've probably stopped using the iPhoto option in Open boxes already just because of its poor performance and awkward sizing.
6) Assuming Siri comes to OSX you should be able to both speak and type to Siri. Maybe integrate this with Spotlight? It would be handy to just hit the shortcut key and type "add reminder blah blah blah" This would be almost like a cross between Siri and Alfred/QuickSilver.
7) Notification Center should probably move to the top of the display on OSX. There's nothing too wrong about having it stashed away on the right hand side of the display but for consistency it just feels kinda weird to me. Instead how about accessing it via hovering your mouse at the top-center of your display and clicking on a little grabber that appears? I'm not a huge fan of these hover / hide & seek ways of accessing things but I'm not sure there's a better way to do it without totally rethinking how the OSX menubar works. For discoverability leave the top right Notification Center icon there too.