I would like to know what I should expect that will not be transferred from my external hard disk, during migration.
One question is 'what is the most' I can migrate (or what elements are included in the migration), but mainly I also want to know the usually undocumented 'what the migration omits'.
An generic answer like 'everything else' would not help much.
It may take me days, to fully explore everything in the new system, transferred with the help of migration assistant.
So it would be helpful to know what should I expect, and what it is not going to be transferred to the new system in any case at all.
Sorry I can't be precise, but I just this myself late last week (using a Time Machine backup). If there's something that wasn't transferred that ought to have been transferred, I haven't found it yet.
It'll transfer all User accounts - every folder.
It'll transfer all apps, applicable system preferences, desktop layout, all the junk lying around on the desktop, keychain....
Basically, when my new machine rebooted, it looked just like my old machine (other than the step-up from 21" to 27" display). Everything was where I expected to find it, from menu bar to dock (including the aforementioned mess that is my desktop).
It does NOT transfer the operating system, or hardware-specific configurations (it's going to setup for the mouse you have, not the mouse you used on the old machine). As far as I know, it does NOT duplicate partitions - Setup Assistant only handles the startup volume. You'd have to create the additional partitions on the new machine using Disk Utility, and use another method for transferring the contents of those other partitions (target disk mode, for example).