But cherry picking questions to prove a point is silly. However, to show it's not a gangup on web apps here we go:
Speed of the install process? Probably slower than loading a web page for serious applications
How does the update system work? Depends if you're releasing as a single statically compiled program or using shared libs that can be updated. Also if you have a db to sync this will affect things.
How quickly can you release them to all platforms? As long as it takes to compile to all compatible targets.
Can they be easily customized and modified by the user? in what regard? if you mean configuration then yes. If you mean being able to manually tweak the style of the application like when fiddling around in the element inspector, then no unless you are using a theme parser that lets them adjust the themes.
Can they be easily shared? yes
What's the permission model like? Depends what granularity you want to have. Permissions can be restricted to the action level, user level, group level, machine level, global level, etc. Whatever logic you want to implement really.
How much can that application access? access in terms of what?