Run an update of 60 packages, browse the internet while waiting for it to complete, find a .deb you're curious about and try to install it. You can't. Regardless of dependencies. Apt uses a global lock. That is bad UX, whether you acknowledge it or not.
How often, other than when you run the first large system update, is this a real issue?
I mean, I'm all for the *.deb to automatically queue when clicked, as that would be nice, but I do not see why I would particularly want it to mess with the system while the system is being messed with.
Also, I could always virtualise if I want the functionality of having an explicit split between system and userland.
Well, the criticism was in fact "you can't install software while updating your system," and it's true - if your system is updating, you have to wait to install another package.
In practice, I don't see this as a major problem, but on the other hand, many people have slower connections that me.
It'd be nice if the apt clients could download the packages without locking the database, and let you select and queue packages even if others are already being installed.