"Explicit is better than Implicit" is a great principle for programming language design, but a user-friendly user-interface plays by different rules. In UX design, actions should be intuitive, and what could be more intuitive than tapping the background to get to the background?
If you think there needs to be a text or icon to hint for this action as well, ok, maybe. But it should be way more subtle than in the mockup. For me, the Application spread is one of the most distinctive and beautiful features of Unity 8, we can't f*** (mess) this one up.
A go-to-desktop-app in the spread... I don't know. Being able to just tap the background feels way more practical to me. Either the go-to-desktop-app would always be in the end of the spread, which would cost you time, or it would be somewhere in the list making it more difficult to find an application you want to switch to. Why add another UI element, if there's already a perfectly good option we can go with that doesn't bloat the UI nor reduce ease of use of the existing usecases?
On the topic of what is home, might i add:
Don't kill the desktop: A manifesto
In my opinion, Unity 8 should become a proper desktop environment, and that includes being able to put stuff on your desktop. Doesn't have to be applications (unless the user wants to, then let them do it), but it would also be incredibly useful for short-lived files. For example, i'm very often travelling by train in Germany, and since we don't have a ticketing app for Ubuntu Touch, i always put the ticket-pdf on my phone and open it in document viewer. That's something i would put on my desktop. Yes, i know, Gnome removed that feature a while back. But for many people that is a standard feature of Desktop Environments and it would be foolish to dismiss it as a legacy usecase. For me, it is clear. The desktop is my 127.0.0.1. And i'd love an easy way to get there.