I dunno about Siri; personally I find the dictation feature in Mountain Lion to be a poor implementation, as all Macs should be capable of doing the processing locally, rather than having to ship it off somewhere. Particularly since Macs had pretty good local voice recognition for a very long time.
If they could make it some kind of local feature, and also let apps include plugins to extend it then it could be a truly killer feature. Siri as it is on iOS I could take or leave; it makes sense for the mobile platform but just isn't good enough for a full blown computer.
CoreStorage
Personally I want to see more from CoreStorage; support for pooled storage, along with integrity checking, error correction and redundancy handling. I could see a Time Machine 2.0 being brilliant if external drives were simply added to a pool with the internal ones, with block-level snapshots providing fast historic backup data, and the Time Machine interface managing it all… would be a dream come true.
Also some extras like CoreStorage compression (handled just like encryption), and the ability to designate faster drives as read-caches… it'd allow CoreStorage to be a properly serious contender as a volume manager. Plus it would allow Apple to then switch to a new HFS Lite, focusing on implementing a modern file-system with native hard-linking etc., without having to worry about anything that CoreStorage can do for them instead.
Usability/Window and App Management
Proper improvements to Mission Control and Fullscreen are a must; fluid support for multiple monitors (with gestures) and a smoother mission control experience so we can get the benefits of grouping while still being able to quickly drill down to the window we need.
The whole area of window switching needs more attention and more refinements to make things even more touch friendly, but also keep things friendly for us non-touch users; special gestures are all well and good, but many interfaces shouldn't need them in the first place.
LaunchPad could use some more innovative improvements. Personally I find the current grid very wasteful and not all that useful. What would be even better would be if it displayed icons at bigger sizes the more frequently used they are (think like a
tag cloud). And even go one better by grouping icons so that apps that are frequently used together will be closer. All to make apps easier to get to, without the need to swipe around at all. Organised, swiping pages would still be needed, but with the tag-cloud style jumping off point your system would "learn" what you use the most and make it easier to get to.
Consistency
A return to uniform apps would be on my wish-list as well, though seemingly very unlikely. I absolutely hate the custom appearances for Notes, Reminders etc., as it completely kills the idea of a uniform, consistent interface. I absolutely blame iTunes for that, but while flashy appearances might be a nice novelty, I'd rather have usability and consistency become the focus once more.
Finder
Redesigned Finder! I still hate the Finder as much I did in OS X 10.0, but at this stage I suspect Apple would rather get rid of it for iCloud instead. Still, there are so many small improvements that could be made to make it so much more fluid. I'd also kill for a combined columns and icons navigation mode, where only the current folder is display as icons while higher levels get shunted to the left as a column. Give it a more touch-friendly sizing on the lists and it could scream as a touch interface.
Dashboard 2.0
I'd also love to see a big revamp to the Dashboard; widgets in the Mac App Store, a Resume like feature to allow them to free up resources when not in use, perhaps combined with notifications/events. Return to the interface being an overlay by default rather than the current, completely useless, default of it being its own Space/fullscreen app. Perhaps even make a focus on Dashboard widgets "docking" with the edges of a screen, to make the Dashboard overlay more content-focused, keeping widgets as the mini-programs that supplement your main apps, rather than distracting from them or simply being different.
When it comes down to it, widgets could be like notification centre on steroids, giving you a custom tailored view of your system, while also giving you quick access to useful tools. A concept of spaces within Dashboard, either giving you separate dashboards for each space, or the ability to swipe between a set of dashboards could be incredibly useful, allowing you to swipe from your social overview dashboard to your programming tools dashboard etc. It's a feature that I was glad to see when it arrived, but have been sad to see it so neglected ever since.
