Also, with the Dock hidden, you don't know where your click target is until the mouse arrives at the relevant screen edge and you've waited for the icons to slide in, at which point you still have to locate and click the icon you want.

It sounds like a small thing, but it annoys me enough that I never hide the Dock.

I know it's not "at a glance" if it requires a key combo, but you can hide and show the dock with Cmd-option-d.

If you're also just wanting to see what's running, Cmd-Tab like you were going to switch apps, but don't cycle it, it shows the running app list.

That's an interesting point, and thanks for the key-combo (I didn't know that). But what I like about being able to see the dock is that I often randomly notice that too many apps are running, without specifically looking for it. It's sort of like when you notice the task-bar getting cluttered with windows in Windows... I probably won't remember to hit a button to see how many apps are running, but occasionally I'll just notice the clutter.

There's also an annoyingly perceptible delay before the dock begins to appear, and until the animation finishes.

Yes. Also: Badges.