I love Apple. By and large, their products are the very model of good design: Simple, intuitive, and elegant. However, even after eight versions, they've completely fumbled with iTunes; almost every time I use it, I find something new to dislike about it.
First, simply from a gestalt point of view, it's a mess. Some functionality is in the menus (including the dreaded Advanced menu), and some is hidden behind buttons bearing cryptic icons, many of which provide little clue what they do. Within the iTunes window, these icons are all over the place; when I need to perform a task, there is little affordance as to where I should look for the widget that makes it happen. The functionality of a music player is intermingled with that of a music-library manager in a most haphazard way. And the Podcasts pane is even worse than the main one.
Playlists are simply a broken model for how to organize music. Consider my family: My wife, my oldest daughter, and I each have an iPod. To try to subdivide our music library into songs that belong on each iPod, I have to have a playlist for all of us, one for my wife and I, one for my wife and daughter, one for my daughter and I, and one for each of us alone. When my two youngest girls get iPods, this is going to exponentiate; I won't need all 31 possibilities, but the number will at least double. And this doesn't even cover the case of subdividing by mood or genre. The right model is tags. I could then specify that, for example, my daughter's iPod should be stocked with songs tagged megan first, and the remaining space filled with (joe and not explicit) (she likes a lot of the same music I do). I know some of this can be done with smart playlists, but it wouldn't be easy and simple and effortlessly "just work" like it could. Also, this would further allow visualization and manipulation of my collection via tag clouds and similar devices like you see on tag-based sites like Flickr.
Initially building a library is always an unpleasant experience in iTunes. Recently I installed a new hard drive for music. I reset iTunes to point there, and asked it to consolidate my library, copying songs from anywhere on the machine to the new drive, and organizing them its own way. This took forever, and many hours into it, I got a dialog informing me that the drive was full, and that iTunes would point to the song in its original location. The buttons on this dialog are OK and Cancel. Presumably OK just acknowledges and dismisses the dialog, but what does Cancel mean? Don't add this song to the library, or abort the entire consolidation? So many hours in, I don't want to abort the whole operation, so I hit OK. Now I get the same dialog again, presumably for the next song (it doesn't tell me what song it is complaining about). I hit OK again, and again, and again. Finally, frustrated, I hit Cancel, but I continue to get these dialogs a bunch more times, and then they stop. I can't tell now if I aborted the consolidation, or if it finished. I see no recourse other than to erase everything and start over, and to hit OK for every one of those dialogs (there ended up being a few hundred of them). There is no "Yes to all" or "Don't show this again" option.
Part of the reason the disk is full is duplicates: Songs that appear in multiple places on my machine. For some reason, iTunes copies and inserts in the library every copy. I can think of no reason to include duplicates in a library, especially when iTunes is making its own copies of the songs. Mind you, I'm not talking about songs with the same title or filename; I'm talking about songs that are bitwise identical on the disk. I don't want these taking up disk space or appearing multiple times in my library (though you can turn off the latter), but I know of no way to avoid it.
Similarly, the dialog you get if you try to add songs to a playlist that are already in it. You are asked whether you want to add them multiple times, or to skip the duplicates. They should have done this the Apple way: Simply don't enable the rare corner case where someone might want the same song in their playlist multiple times, and skip them always. Or at least give the dialog a "Don't ask this again" option.
As an extra added bonus, one of its automatic upgrades deleted all of my ratings data.
It's not just me; if you Google "itunes sucks" you get almost 18,000 hits, which cover most everything I've complained about here and many other equally legitimate issues.
I'm going to move my whole collection to a Linux box running some open-source music manager (I haven't figured out which one yet). I hardly ever buy music from iTunes anyway, because I can get it DRM-free from Amazon without paying extra for the privilege. I'll miss the free songs from Starbucks, but not enough to put up with this piece of garbage any longer. Given how hard it is to move a music collection to a new platform, you can guess how likely I am to ever come back to iTunes, even if they make it awesome. Congrats, Apple - your sorry usability has lost you a customer from the iTunes ecosystem for life. It'd be shameful even for a company who doesn't have such a long history of great design. Maybe I should send them my copy of Design of Everyday Things; apparently the iTunes team hasn't read it.