Why can't you put everything on the web? Right now it is not powerful enough for 3D rendering but that is mostly a limitation of Javascript and people using old browsers.
Other than than I don't see any program which is used by ordinary people today that you couldn't put on the net. Most of them properly already should have been.
Why would I want to? I really don't see the point of trying to run a real time industrial automation control from a javascript engine inside fire fox. It might be a fun hack but I fail to see any real use.
Well obviously you need someway to write to the physical wires and if true real-time is required you will have to skip Javascript, but I can't see the issue with having the user facing part of the system be a webapp -- in fact it would properly be easier to do and would be safer too since the system is better walled of from things like viruses and malware.
Other than than I don't see any program which is used by ordinary people today that you couldn't put on the net. Most of them properly already should have been.