Improved Syncing — It used to be that when you added a bunch of music, videos, photos, and apps to your iPad or iPhone you were likely to get some sort of message saying it couldn't be done for lack of space. This was infuriating because iTunes knows how much data you have and it knows how much space you have on your device. Why not tell me there isn't enough space BEFORE you sync? Well, iTunes 10 now does that, only better. Real-time updates on available space as you add data.
Playlist List — I might be the only one who had this issue, or the only one who was ENRAGED by it but it's like a dream come true for me. This playlist menu has bugged me for years. Scrolling was painfully slow and unusable with the scroll-wheel (or finger-scrolling?). Picking through a couple hundred playlists to fill my iPhone was excruciating. With iTunes 10 this simple menu now behaves like it should.
Great job Apple. Once again.