Things that I think will change as of WWDC 2014 that nobody but a very few developers are likely to care about, although the implications may be fairly wide ranging:
64-bit Mac App Store submissions
Mac App Store submissions for new apps and upgrades other than maintenance updates will have to be 64-bit only. Any executables containing
i386code will be rejected automatically during submission. Maintenance updates to already shipping applications will be grandfathered in.
64-bit only Mac OS SDK
The 10.10 SDK will no longer include any 32-bit frameworks or libraries. This includes not just old frameworks such as
HIToolboxthat were never upgraded to
x86_64, but also
i386architectures in all other frameworks. Building maintenance updates or modernizing legacy code will require using an older SDK. (10.8/10.9 SDKs will probably continue to support
i386, but we will probably see the 10.7 SDK dropped at the next Xcode revision).
Assuming that the rumored sweeping changes to the overall look of the OS (and, more importantly, to the metrics of various UI widgets) come to pass, applications which are built to deploy on 10.8 or later and which use autolayout will get the new appearance. Applications built against older SDKs or which do not use autolayout will get the “legacy” (that is, 10.9) appearance.
This will work against a consistent look and feel across all running applications, but may be effective as leverage to move developers toward the current best practices.
(This does not mean that 32-bit applications will suddenly stop working, because that isn’t the case. Existing applications will run on 10.10 as well as they did on 10.9, with the usual caveats that come with a major OS upgrade.)