It is getting an ‘end-to-end’ redesign - just as its iOS received last year – that will make Mac and IOS look similar and offer better interoperability. Although the Cupertino based Apple is coy about whether future Macs will go touch as well!
The new OS X has an internal codename ‘Syrah' which is commonly known as Shiraz – a dark skinned red grape variety – which makes about as much sense as naming it after laid back Californian beaches or ferocious members of the cat family.
According to 9TO5Mac "The new look will have similar toggle designs to iOS 7, sharper window corners, more defined icons across the system, and more white space than the current version. The [iOS] convergence will solely surround feature parity that makes sense and aesthetics."
John Gruber, on occasions referred to as "Apple apologist", “Apple's favourite blogger" or just "fan boy" blogged that this was "The most dramatic OS X redesign in history." High expectations and something Apple needs to take great care with – Mac users do not like quantum change.
New iOS too
Rumour is that iOS 8 may miss the developers' conference – although I cannot imagine Apple doing that but word is that iOS 8.1 is the real release coming later in the year with the iPhone 6.
iOS 8.x is not a quantum leap. Apple is working on
- a Healthbook application for aggregating various health statistics from App Store apps and third-party medical/fitness devices
- an updated Maps app with public transit directions support
- a standalone iTunes Radio application to boost usage and recognise songs
- VoLTE calling support
- TextEdit and Preview apps
- Siri enhancements
- and various enhancements across the system
It is likely that it will not support iPad generation 2 or 3 and may not support iPhone 4/s – if it is to throw off its 32-bit shackles (although iPad 4 is 32 bit so we cannot verify this rumour).