Lack of auto conversion support when transferring to devices.
"Genius" Playlists never work for anything obscure
Ping is terrible
Lack of tagging support (no autotag feature)
I use mediamonkey (i'm on PC), once you switch you can never go back. I can organize my collection exactly how I want it from in the software, i can browse my harddrives to preview music before I add to my library. It supports pretty much every codec under the sun AND will convert it to compatible versions for whatever device i'm syncing to.
There is also a plugin for discogs tagging support (one of the best online discographies in the world), a plug in for "autodj" (based off last.fm).
I realize that not everyone in the world is as anal about organizing music as I am, but for the job, itunes can't compete.