Hi!
I believe we can make installing a fully-featured MediaWiki service system as simple as copy&pasting 2-3 lines to a shell, or even executing a remote
I think this is underestimating the issue, unfortunately. I.e., in ideal situation - the user runs the same system, with the same services and permissions, that the shell lines were tested on, it probably would work pretty good. But what if he doesn't? What if he doesn't have root access? Has non-Linux Unix system? Has custom-built VPS system with custom-tailored access/capability permissions? Has Windows (not talking about more exotic systems that do have PHP support)? Has no direct Internet access? Has security settings that further restrict their capabilities? Has some of the libraries/packages that our dependencies' dependencies installed in different versions that support different options/APIs/command-line parameters? The problem would be not in those 3 shell lines, but in the complexity those lines hide and in the assumptions those 3 lines make. The difference is between "if this system runs PHP, you're OK" to "if this system can run this three scripts which runs a dozen of scripts which invoke a dozen of software packages with an unknown number of dependencies each and all of them are OK then you're OK".
The point is more moving parts you add, more potential failures are there, and these potential failures very soon become real ones when users start using it in a wide wild world.
And my concern here is that the users - and, more importantly, potential contributors - faced with these failures, would do what James mentioned - pack it and move on to something else, which still is "if you have running PHP, you're OK".
installer script that runs those lines for you. Additionally, we can offer VM images derived from the same install process through Bitnami or others.
VM solves the dependency/environment issue, but running VM on a constant basis requires some sysadmin skills and knowledge (and much higher hardware/access requirements) which again means people with no sysadmin skills would be cut off.