As with any new software, there are bound to be some initial problems people experience, either with the software itself or with compatibility with other software and hardware. A few of these are surfacing and they include problems with media servers, music not syncing with iPhones, and music not importing from various sources.
Many people have massive iTunes libraries they store on media server devices such as an external RAID array attached to a computer running media server software like FireFly. While these servers are technically supposed to work with iTunes, there may be some small changes in how the protocols are handled which are preventing them from working with iTunes 10, at least until an update fixes the issues. This could be because of problems in either the server or iTunes, and updates to both may be required for full functionality to be restored.
iPhones not syncing
Some people who have tried syncing their iPhones after upgrading to iTunes 10 have found the program will change its status to "Synching iPhone" or "Preparing to Sync" and then just sit there or go back to the normal iTunes window without doing anything. The phone ends up with no music or other items on it.
If this happens, you can try resetting the iPhone and optionally resetting the sync services on your Mac. To reset the iPhone, go to the settings and select "General." Then go to the "Reset" section and choose the "Reset All Settings" option. When this is done, connect the phone to your system and try to sync the music and other data again.
To reset the sync services in OS X, follow the instructions outlined in this Apple Knowledgebase Document.
Music not importing
Some people are having issues importing CDs to iTunes; the program will act erratic and switch views between the CD and the iTunes library. This is clearly not a desired behavior. Additionally, others have complained that iTunes will not add items to the iTunes library when they are dragged to the iTunes window.
This could be an issue with the location of the iTunes library, so be sure to check the "Advanced" section of the iTunes preferences and ensure the library being referenced is available to the system.
Some of these and other problems may be caused by conflicts with configuration files and other resources in the current user's account, or could be from bugs in the program. To test for account-based problems, create a new user account and try importing, syncing, or connecting to media servers from that account.
If the problem is somewhere in the user's account, some approaches that may work are to try removing the iTunes preferences and relevant settings files. For iTunes, these are the "com.apple.iTunes.plist," "com.apple.iTunes.eq.plist," and "com.apple.iTunesHelper.plist" files located in the /username/Library/Preferences/ folder. Additionally, you can try removing the CDDB settings files ("CD Info.cidb" and "CDDB Preferences") from the same preferences folder, which may help if you are having troubles with audio CDs. In addition to the standard preference files for iTunes, there is an additional one in the /username/Library/Preferences/ByHost folder which can be deleted.
As mentioned before, resetting sync services may also help some problems, especially if you have issues with devices like iPods and iPhones. In addition to resetting the sync services, you can try using a maintenance utility like OnyX, MacCleanse, or MacKeeper to remove user caches and other temporary files.
If iTunes is still unsuccessful when testing in a new account, then you can look at more global options like iTunes-related plug-ins, utilities, or other add-ons. Try removing any third-party add-ons, and booting to Safe Mode to try some of the iTunes functions without third-party extensions loaded. You can also try reapplying the latest Combo updater for your version of OS X, to see if that replaces any corrupt system files that could be contributing to the problem.