Eject a disk by moving it from my desktop to the trash with all the files I want to delete? Makes sense.
Well, to understand this, you have to recall that early Macs had to be able to run off of a single floppy drive. Users might buy a hard drive or a second floppy drive (or if they had a dual-floppy SE, a third floppy drive for some reason) but it couldn't be relied on. Yet they still had to be able to tolerate having the OS disc ejected at times.
So there was a distinction between physically ejecting a disc while keeping it mounted (which was represented onscreen by a greyed out disc icon) so that you could copy to it, and both physically ejecting _and_ dismounting a disc.
The formal way that you were supposed to do this was by using menu commands. The Eject command was for eject-but-keep-mounted while the generally ignored Put Away command was for eject-and-dismount. It was also possible to use Put Away on an already greyed out, ejected-but-mounted disc icon.
User testing showed that this was inconvenient, and one of the OS developers eventually created a shortcut for the Put Away command, which was to drag a disc icon to the trash. It wound up being so popular that it shipped.
Apparently there had been some thought at the time about changing the Trash icon into some sort of Eject icon in the case of ejecting a disc, but apparently this was felt to be confusing or too difficult, so it wasn't done. In OS X the idea was revisited, and now the Trash icon does turn into a standard Eject icon when you're dragging a disc.
In any case, in real life, whatever confusion dragging disc icons to the trash might have caused, everyone got over it basically immediately.
Switching tiled applications makes the one menu bar change? Sure. It's not like moving the cursor half the screen for each click is a waste of time.
It's not; since there's nothing above the menubar, you can just slam the mouse up. It turns out to be faster and easier than having multiple menu bars. The Mac and Lisa groups did consider per-window menubars, but having tested the idea, it was rejected. For example, here's some polaroids of a screen from 1980 showing a Lisa with a menu attached to the bottom of a window: http://www.folklore.org/images... Later that year, the menu had moved to the top of the windows: http://www.folklore.org/images... And early the next year, it finally settled at the top of the screen: http://www.folklore.org/images...