I can see you are a deep thinker Mike and with some neat ideas too! I have been looking into a lot of security based ideas recently, and even made some changes to my filesystems and browsers as a result, but nothing really does what I want it too. Ideas like SeLInux and AppArmor are way way too complicated for anything other than corporate users with sysadmin backup, ideas like Tor are fine if you are a paedophile or a dissident in a country without any concept of free speech, but otherwise are really OTT, but ideas like separate user accounts for separate functions are really much nearer the mark.
I like xenopeek's Qubes recommendation as well - I haven't heard of it before, but unfortunately anything based on virtualisation instantly turns me off, even though I use Vm's occasionally, I have always thought they are dramatically over hyped, a bit like "The Emperor's new clothes" and I would prefer not to use them at all, but I do understand their security potential. I still might have a look at Qubes though.
The only problem with your idea Mike is that I think you would have to tailor the Linux file system permissions from 'ugo' (user, group, others) to 'u1,u2,u3 etc g1,g2,g3 etc, others' to make it really work and that might not be too easy and would play havoc with newbies
Edit. Actually that last sentence is complete rubbish, you can easily accommodate what you suggest with existing groups permissions.
Fujitsu Lifebook AH532. Intel i5 processor, 6Gb ram, Intel HD3000 graphics, Intel Audio/wifi. Realtek RTL8111/8168B Ethernet.Lubuntu 13.10,Ubuntu12.10 (Unity), Mint16 (Cinnamon), Manjaro (Xfce).