Having file-format-specific reading utilities is stupid, awful, and is precisely the type of Windows-centric (and to a lesser extent Mac-centric) behavior I absolutely loath.
Applications centered around tasks however are a bit of a different story, and that's more of what I'm describing.
For reading, Adobe's an absolutely horrible example. Particularly on tablets. Don't make me remember the time I was buried to my waist in a colo cabinet trying to sort out load balancer issues while reading the 300+ page manual on my Android smartphone using the Adobe reader app ... which would reset to the front page each time it got kicked out ... which is precisely what was happening as a recruiter was calling me despite my repeatedly hanging up on her (and having net nil reception regardless). There are some modestly better PDF readers (say, evince), though must fail on the basis of not positioning the text optimally for reading.
Contrast with a stunning exception to the usual rule that online readers are crap: the Internet Archive's book reader. I discuss it briefly here: http://redd.it/1w0n83
The beauty of it? It autocrops the page to the visible content on it. Screenshot: http://i.imgur.com/Reg8KLB.png
You can further maximize the browser (F11) and remove the navigation elements so that _all_ you are seeing is the text you're reading. Page navigation is quick and intuitive. The entire thing is, incredibly, better than any desktop PDF viewer I've encountered.
What I'd really like is something somewhere between Calibre and Zotero: that will manage a selection of documents, organize and manage them, spawn viewers (preferably good and useful viewers -- Calibre on Linux fails massively in this regard). And, if I specify it, renders everything with minimal markup.
As for shopping: it's not that I'd fire up the Amazon app, I'd fire up the shopping app. You want a standard, uniformly designed client with solid security, not a mash of individually created apps, each with its own security flaws and excessive permissions.
Splitting shopping from web-browsing would also prevent surveilling users across the Internet from the shopping interface itself.
For specific application-based tools. Some sort of general app framework that could be fired up. The main distinction between it and the reader would be that a reader app would assume that it's valid at any time to dump state to disk and bail, whereas you could configure an app for how you wanted it to behave (I might want a monitor to be up 24/7/365, while a social networking app could shut down if I haven't interacted with it for 15 minutes).