Wouldn't it be great if Apple decided to just put all of iTunes in one folder so it could be backed up and moved as needed?
For the most part, this is exactly how iTunes has worked for years. (The exceptions are preferences and iOS device backups.) In a default iTunes install, the all-important "iTunes Library.itl" file -- the catalog of all content files that have been added to iTunes, including apps -- lives inside the "iTunes" folder, as does the record of all CDs that you've ripped (iTunes Library Extras.itdb), results obtained from Genius (iTunes Library Genius.itdb), art downloaded from the Store and art cache for speedy Cover Flow display (Album Artwork folder), and copies of "old" library files automatically saved at the time of each iTunes update (Previous iTunes Libraries folder) to facilitate "rolling back" iTunes in the event of a truly disasterous release. Last but not least is the "iTunes Media" folder into which a default iTunes install will copy every content file you add to iTunes (and, in the case of files that iTunes creates, such as CD rips, Store purchases, and format conversions, is the place where such files are first created).
When you use the defaults, moving iTunes to another computer is exactly this simple:
1) Install iTunes on new computer; launch it once just to let it get set up (create a prefs file, create an empty iTunes folder, bug you about looking for music files, etc.); quit iTunes.
2) Make a copy of the "iTunes" folder from the old computer; use that copy to replace the just-created "iTunes" folder on the new machine.
2a) If you make purchases from the iTunes Store, or use AirTunes, or want to use Home Sharing, then iTunes Store menu->Authorize This Computer, and provide your Store login
3) There is no Step 3; you're done. The first time you connect your iOS device to the new machine's iTunes, a new backup is generated and you're all set.
With the exception of preferences, your new machine now has everything that was on your old machine -- all the tracks, all the apps, all the playlists, counts, ratings, art, etc. (crashtestdummy's method, from a few days ago, of using the "iTunes Library.xml" to effect a transfer was...overthinking. By the time I saw his post -- and its followup -- it seemed too late to save him the extra work he wound up having to do; sorry about that crash! Plus, I was moving households, and time & temper were short ;-)
With a default install, the above method works even when moving from one platform to another -- Win to Mac, or Mac to Win -- with the small added step of adding or removing the ".itl" file extension from the library file's name. It really is dead simple to do.
Most problems arise from folks wanting or needing to do things their own way. Wanting: "I've been storing my music files in my own directory structure since 1990, and I'm not going to give in to Apple's system!"; or " Apple's storage system makes it hard to easily 'share' all the files in my favorite playlist with my 100 best friends"; etc.
Needing: boot drives are too small to handle large music collections, so music files have to be offloaded to external hard drives; or trusting all their music to storage on an iPod and then having the iPod crash; or their old computer dies and they've got no backup; etc.
In those cases, transferring an iTunes install to a new computer usually requires Consolidating files, or emergency measures, or both. Consolidating wrecks peoples' private storage schemes; much anger ensues. Or the emergency measures cost money or time, which somehow is never the user's fault ;-). Or...well, take your pick -- these boards have p'bly seen every variant of issues arising during transfers. (When properly set up, custom folder schemes or other non-default behavior doesn't always require consolidation in order to transfer, but Apple doesn't publish anything about it and no one thinks to ask at places like iLounge before they jump in headfirst....)
Preferences have to be moved by-hand in order to be retained across a transfer, and that can be a small PITA, especially for Windows users (the filepath of the Prefs file keeps jumping around from Win XP to Vista to 7), but iTunes is kinda blameless on that one -- it tries to play nice with the conventions of the OS. Ditto for the location of iOD backups. But even then, you just locate the file on theold machine, copy it to the correct spot on the new machine, and Bob's your uncle. (But really, it certainly wouldn't kill Apple to create a simple tool that could bundle those things up and drop 'em into the right locations on the the new machine.) But overall, if you simply let iTunes do its thing, moving to another machine is a breeze.
It's tough to make everyone happy. On the one hand there's iTunes, asking folks to adopt its particular "zen" in exchange for ease of use; on the other hand there's MS Word, which stockpiles so many special-purpose features that "average" users are regularly overwhemed by features....