I like the idea of repositories and a central place for installing, updating and deleting stuff but there is one big downside on it:
Little update = big download.
I also like the idea of apt-get (or whatever flavor your prefer) style download and installation of software. but i 'd rather not translate the unix way of doing this in a one to one fashion. i think if someone created a central software repository where all software is packaged with msi, patches are provided with msp etc.. and the (command line!) client tools are basically talking to this repository service and making calls to the windows installer api.. that could lead to some interesting scenarios, i think.
the msi patching mechanism already allows for "difference" patching (on a coarse level, yes). i think something like blastwave for windows using msi/msp technology could be done and would be extremely beneficial to users (those who like the cmd line at least).