Apple will stop the further development of 32-Bit APIs. 32-Bit software will still run, but new APIs or features will be 64-Bit only. In 10.8, 32-Bit support will completely disappear.
There will be more corporate features, as for example a better integration with Microsoft Active Directory.
There will be more support for multi-touch screens or other input devices.
I think it's very likely we'll see the end of 32-bit support in 10.7, thus shutting out Rev. A MacBook and MacBook Pro's. I think we will also see the removal of Rosetta entirely on 10.7.
I don't know how, but somewhere along the way Apple became less friendly to older hardware; my 10-year old G4 could be upgraded to 10.4.11, yet 4-year old machines can't run 10.6, so something has definitely changed.
I think we will see a further merge behind the scenes of iOS and OS X, e.g. CoreMotion in 10.7 is likely. 3G-equipped notebooks would be something to think about as well.
I don't think the UI will change very much. In 6 versions of OS X, and 4 versions of iOS, the UI has changed little. 10.5 was a huge makeover, but some things, such as progress and scroll bars, date back to 10.2. If we get a new UI I will be surprised.
Blu-ray is never going to happen; in fact, I think we'd see the removal of optical drives from notebooks happen first, and I think this is the real reason behind no Blu-ray support, as Jobs plans on doing to DVDs what he did to floppies with the original iMac. It's possible that in 2 or 3 years, no apple product, except the Mac Pro, will have an optical drive.
If any of that happens, I will never buy another Apple product.
I agree. I don't think Apple will replace OS X with iOS, nor would they put iOS on Macs. Instead, I think we will see a gradual downsizing of Mac offerings and expansion of iDevices. For example, it is possible that Apple's hardware line-up in 2015 looks like this:
Macs:
Mac Pro
iMac (one SKU)
MacBook Pro (only 17", other two discontinued)
iDevices:
iPhone
iPod (by this time, only iPod touch and iPod nano remain, nano goes to a touch interface)
Apple TV
iPad: 11", 13", and 15" versions
Just off the top of my head. The Mac is on it's way out.