I’m well aware of strategies using chroot, virtual machines, and the like. These are useful tools up to a point, but a long way short of what I would ideally like to see. For example, they restrict access at a very coarse level compared to the kinds of user/group/ACL models we use in many other contexts. By their nature, they also do not admit convenient ways to break out of the jail with the user’s explicit consent. Once you get beyond individual applications with their own dedicated file types and consider more generic cases like text editors working on text files that are likely to exist throughout a filesystem, this lack of flexibility is a serious limitation. Another key distinction is that chroot and the like are voluntary mechanisms, usually off by default and therefore not completely enforced by the OS.
Some systems mentioned in this HN discussion are closer to the kind of model I had in mind. As other posters have pointed out, the difficulty is how you structure a system so that it is reasonably effective by default but still usable by non-experts. I believe we could achieve this — or at least get much closer than the typical security models we use at the moment — but it will surely take a lot more thought and experimentation than we have attempted as an industry so far. Microsoft’s UAC mechanism makes an interesting case study here: it was fundamentally a reasonable idea, but the first implementation proved too intrusive for average users to tolerate and lost much of its effectiveness as a result.