Having tinkered with the iOS and Yosemite betas for the last few weeks I've had some experience of the upgrade and therefore the problems that might be encountered. The thing that is going to affect the most people that I can reasonably predict relates to iMessage. I think this is due to Apple's fix to a problem some ex-iPhone users were seeing where long after you gave up your iPhone iMessages would still gobble up any texts an iPhone using friend tried to send to you.
Now when your iPhone updates it appears to unregister your phone number with iMessage, which automatically unregisters your phone number in iMessage on all of your devices (which is very sensible). If you use your phone number as your primary iMessage address (which I generally recommend for simplicity) then it will be deselected on your other devices when it is unregistered and one of your emails will be selected instead.
When your phone upgrade is complete your phone number will be re-registered with iMessage on your phone automatically but this will not necessarily be reflected in iMessage settings on your other devices. My recommendation is to make a point of double checking your iMessage settings after upgrading your iPhone to iOS 8, and if you have multiple devices, upgrade your iPhone first of all.
There are some great improvements to iMessage in iOS 8' but setup and configuration in my opinion is still cludgy and would benefit greatly from settings synchronisation.