ArmA has astonishing modding capabilities. However, adding mods to a public server means raising the entrance barrier to a point where almost nobody will join unless the set of mods is extremely popular.
Tools like "Play withSIX" simplify the process of downloading and installing required mods but only an integrated solution can make it really smooth for everyone.
I suggest that by default missing mods are downloaded along with the mission file and/or temporarily enabled when a player joins a server.
To take load off of the game server it would be nice to be able to specify an alternate "fast download" server that hosts the files.