no big upgrade (at least not from the users stand point). It is mostly a big collection of tweaks etc. OS10 has been 64 bit for a while, nothing new there. I guess the big upgrade that appeals to some users is the addition of microsoft exchange support throughout the OS. The four things they are advertising are:
1)"Better.Faster.Easier"-It enhances your mac experience, haha how specific. http://www.apple.com/ca/macosx/refinements/
2) "Next-generation technologies"- basically saying we added some shit for developers that you are too dumb to understand. We won't even bother explaining http://www.apple.com/ca/macosx/technology/
3)Accessibility- Bunch of new features that helps the disabled use the Mac. I am actually glad to see them work on this, even though it is a small niche it is a good thing to do. http://www.apple.com/ca/macosx/universal-access/
4)Microsoft Exchange- Basically appeals for people using a mac with work http://www.apple.com/ca/macosx/exchange/
The links explain each point in more detail. For most people though there is nothing new that will "wow" you. Hence the minor upgrade. But for developers it is huge and as a result consumers will benefit with programs that come out.