Ooh boy :) As soon as I saw which snippet they pulled for the summary I knew there would be fun in the comments. So let me clarify for those who haven't been following Plasma too closely in the last year or two.
Plasma Active is for consumer devices, such as (but not exclusively) tablets. Plasma Desktop is for full on desktop systems.
In Plasma Desktop, we have a very traditional approach to the file system workflow, though augmented by Activities which are also optional (you can freely ignore them if you wish). So you have the file system and the file manager, etc.
On consumer devices, picking through file hierarchies is a lot more painful due to input method limitations. The use cases of such devices is also fairly different.
To make this smoother, in Plasma Active, activities are front and center (and not particularly optional, though you can stick with just 1 if you care to) and we use metadata information extensively. When looking at photos, I probably want all the photos associated with a given activity (e.g. a vacation activity, or a party activity, or a project activity..) or with a given bit of metadata (such as a tag, or perhaps a geographical location).
Given how devices such as tablets are actually used with comfort (and not in the limited fashion seen fit by Apple and Google), this tends to work really, really nicely. Of course, we do let you explore the file system hierarchy (graphically or via a terminal if you wish) but that's simply not the first UI you are presented with and forced to use (usually awkwardly on a tablet, phone or t.v. screen).
(Removable media is treated specially again, in ways that will make people here happy I think, but for brevity (ha!) I won't get into that here..)
I'm a "pry my keyboard from my cold dead hands, and give me those overlapping windows while you're at it" guy (mouse is very nice, but I can live without it if push comes to shove ;), so I'm not blindly in love with devices at the expense of a good desktop. None of our team is, in fact.
What's really compelling, I think, is that with Plasma Workspaces 2, it will be easy to switch between the different workspace shells (and their paradigms) at runtime. So your tablet can become a little desktop if you want without any problems .. and you still get to use Plasma and the rest of the KDE software while you're at it :)
In any case, try it before you dismiss it; don't judge Active by Android/iOS limitations; keep in mind that we retain a full desktop env and like it that way :)