I would like the client to indicate if an addon is not enabled for any character.

I'm not sure if the WoW client reads all the files in /interface/addons but I prefer to delete unused addons.

About your first thing, I think that may be beyond the scope of the Client; however, you can, at the char select screen of WoW (don't know about WAR, sorry) click the Addons button. In the new window, you use the pulldown menu to select individual characters, and check/uncheck addons. Also, there is the setting for Load out of date addons, which you should turn on after every patch Blizzard does, as they turn it off every patch, to prevent addons from potentially crashing the game.

The second, in the Client, there are two things: whether the Client installed the addon(s) or they are unrecognized (ex: not hosted on Curse) you can Uninstall them using the Uninstall button.

To remove savedvariables from deleted addons, the Tools button as a "Clean savedvariables" option.

Myrroddin is correct,

The client would have to start altering wow files in order to enable/disable characters (or to view that information), something we are very reluctant to do.