Kirk McElhearn, Macworld:
In a way, this may be a predictable side effect of Apple’s push to online services. The company wants everything to be in the cloud, and it would prefer that you buy all your music and movies from there as well. Local syncing isn’t really a part of that plan and so may be treated as an afterthought. The difficulty is that not all users are right for the cloud model. For those with large iTunes libraries, or with limited broadband bandwidth, cloud storage simply isn’t usable.
My iTunes music library — not counting video, podcast, or anything else — is over 300 GB. So many of the problems I’ve seen with iTunes syncing have manifested themselves or become worse with an ever-expanding library.
I’m sure that my unreasonably large library is only part of the problem though; I’ve seen these and other issues with libraries a tenth the size.