I got hit with this Bug, too. Will list my stats: new iPhone 6 Plus, upgraded to iOS 8.1.2, Mac OS X 10.8.5, current iTunes (12.0.126). Unlike many of the helpful posts here, almost none of the suggestions would apply in my case. I determined that the reason for this status message, in my case, was iTunes was waiting to copy about six Apps that were either incompatible or corrupted. I am still working through completing a full sync, but I'd like to make some observations:
In iTunes, there are very few clues available to us to help diagnose the problem. I note that if you select the device icon, the one that will display in the thumbnail the name of the iPhone, and the capacity, next to the spinning arrows, you will likely also see a warning triangle. That is the only clue I had that the program was actually reporting a problem. Clicking on the icon brings up a window (which you can't resize), which will tell you the error(s) iTunes has encountered, but it's up to you to reach the proper conclusion. In my case, deleting/uninstalling the problem Apps allows me to proceed to the next sync phase.
Apple simply needs to add better diagnostic and troubleshooting tools into iTunes. This Sync problem has been reported since iOS 5, and while it doesn't look like this issue will go away anytime soon, giving advanced users access to better information would remove the 'mystery' behind these kinds of errors and expedite troubleshooting, and maybe even a bug fix from Apple's developers.
- Add a 'Verbose' method to iTunes Sync for Advanced Users. This way you can actually monitor the progress of your sync, and make a diagnosis if something goes wrong.
- Allow the Triangle Error window to be resizable so you can read more about the errors.
- Have iTunes save this error window as a Log, which you can access from a menu, or export later.
- Add a checkbox under the Summary page that will allow iTunes to Skip syncing any file class, such as Apps, pictures, music, etc., so the Sync doesn't get hung up (and add this status it to the Sync log).
- Add the ability to uncheck Apps Sync (unless I missed this feature) so you can isolate this function when troubleshooting sync. You can uncheck syncing music and videos, but not Apps. Why?
- Someone else reported a Diagnostic menu item in an earlier version of iTunes. I don't remember seeing this feature, but if it was there, Apple should put this back.
- Offer a 'repair' option for iTunes internal DBs/backups.
Just my two cents.