Rob Griffiths (of Many Tricks and founder of Mac OS X Hints) had a rough couple of days this past weekend trying to get his new iPhone 6 to sync with his iTunes library. It just wouldn't. He writes about the ordeal in "An iPhone 6, (no) movies, and me".
It turns out, as Rob reports in a follow-up, "A nasty little iTunes/iOS bug may be causing media sync issues", that there's a bug in the pipeline between the current versions of iTunes and iOS 8. The syncing may stop if you have duplicates of purchased media files.
At first, Rob tried using iTunes' "Show Duplicates" tool to locate any duplicates. But it didn't find every duplicate track entry (it's not very rigorous, even using "Show Exact Duplicates"). He then turned to my duplicate managing app Dupin and was able to find the dupes by jiggering the Criteria settings to find album tracks with the same track numbermatching using the name of the tracks failed because the duplicates had slightly different titles. (I'm pretty sure iTunes' "Show Duplicates" always considers the name of the tracks.) Once he eliminated the duplicates the syncing went fine.
I must admit, this doesn't affect me and I hadn't even heard about this issue until now. Of course, it's still early days with iOS 8 adoption.