I find this all very alarming. I don't mind adding app which look similar to iOS apps -- I don't have to use them (even the dreadful Lion look of iCal and AddressBook, which make me avoid ever putting them on a beamer in public out of sheer embarassement in front of Windows folk).
However, I am deeply worried by the implied changes to the file system (I noticed at once that the illustration of Mountain Lion's apps show no sign of disks, folders or files on the desktop even when the menu is showing Finder). Only Martin Pace here has pointed out one of the many things which will break in the app-centric vision.
Like many in niche areas, I work with multiple apps from many vendors. In my spare time I make astro-photos. I acquire image data (dozens to hundereds of files) with a particular software. These files have quasi-identical names, differing only with the optical filter name and a sequence number. These are then calibrated, aligned and processed in another software, from another vendor. Each of these steps is separate and requires segregation of the mutiple new files into new folders to avoid an impossible file-management problem. The images resulting from this process are then imported to another software (PhotoShop) for further processing, and the end results used in yet more software for display.
The image acquisition, by the way, only works because some 4 or 5 softwares are cooperating, exchanging data between themselves and multiple hardware devices -- I have 8 USB->serial ports and 6 USB-driven hardware gadgets cooperating. Most of this will not work under sandboxing.
This is just not going to work in the app-centric model. After holding out for so many years I'll have to move over to Windows. This is such a pity, to ruin a wonderful Unix-based operating system so that people can have a big phone on thier desk.
WhenSteve Jobs first introduced the iPhone, he received a big cheer when he said that it ran OS X. I suspect that had he said that in a few years OS X will be reduced to a replica of a phone operating system the applause may have been rather more muted.
A colleague of mine writes and runs cutting edge synthetic aperture radar processing software on his MacBook Pro. His software powers a big data processing centre which provides processed satellite data to scientists worldwide. I can tell you right now that he would not have been able to do that on an iPhone.
Phones are for doing phone things and computers are for doing computer things. Lets try to remember that.
As for Gatekeeper, it's the obvious next step after sandboxing, and was accurately predicted. And I'm sure that the prediction for the next OS (probably to be called ***** Cat, in keeping with its capabilities) will be that the "anywhere" option will disppear. To be followed in the first update by anything other than sandboxed App Store apps.
But by that time all "power-users" and most serious users will have moved on.
As a Mac user and advocate since 1990 I am deeply saddened.