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I suspect the new API-level technology will be outlined at WWDC this year. Probably for a final release sometime next year.

My thoughts would be that Apple will use 10.7 to leverage a lot of the APIs that have been added in 10.6. Complete, pervasive, use of Grand Central Dispatch (even more than now).

Expansion of QuickTime X to provide modern OO APIs for editing that currently still rely on old-school C APIs and QT7. More codecs built into Quicktime X with an official plugin scheme for adding then (currently added codecs use QT7)

Possibly some generic API that opens up hardware acceleration for plugins in WebKit.
 
I would expect to finally see fully working resolution independent scaling of the user interface. As displays get higher resolutions at smaller physical sizes this will become important.

I hope they also improve the Dock - it desperately needs proper indicators for the currently selected app, number of open windows in an app and stacks could use better manipulation of files (like right click capabilities).

In an ideal world Apple would also just buy Cocoatech's Path Finder and replace the Finder abomination with it but unfortunately that is unlikely to happen...
 
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