Sure, but the interface is probably just as important as the actual logic behind it, isn't it?

The logic is why I love Apt. Most robust dependency resolution algorithms I've used.

But also, I don't have any issues with the CLI. Having a distinction between apt-get and apt-cache and apt-mark doesn't feel weird to me. You're practically just separating the top-level sub commands by a dash instead of a space. The apt command is really just a convenience thing, and there are specialized tools for the more advanced things. Which is fine by me.

Also, the top level apt command doesn't guarantee a stable CLI, so for scripting you're supposed to use apt-get and friends anyway.

Honestly I would consider that one of the fundamental things a package manager must do.

You'd be surprised. Homebrew (the de facto standard package manager for macOS) doesn't do this. Though, you can at least lookup the "leaf" packages which are not dependencies of any other package.

And, most language-specific package managers can't do this. E.g. if you install software with pip or cargo.

you have to remember the extra step after it's finished installing

If the package is in use, it shouldn't be an orphan.

For example, what if you race with a cleanup job that is removing orphans? (Debian is hyper stable, so I often enable unattended upgrades with autoremove. I'm not so comfortable doing that on Arch ;)

What you've described is just an apt-get install when you start and and apt-get remove when you're done. Or more properly setting it as a build dependency in your source package, to let Apt handle it.

But also, why uninstall build tools?

This, at least version constraints, is another one I'd consider essential tbh. The rest are great though, I agree.

Yeah, version constraints are common. But most other package managers bail with an error when they encounter a conflict. Apt is really good about solving conflicts and proposing solutions. Often it will propose multiple solutions to your conflict for you to choose from.

Again, it's the solver part of Apt that makes it the best IMO.