Hey all,
There was an issue which affected our CDN (content delivery network) used for the Battle.net app and all Blizzard games. Our teams are working on it now and we expect it to be cleared up shortly. This is likely causing the stuck on initializing for most players right now (though getting stuck at this stage may be caused by other issues outside of the current CDN problem).
As things start to clear up here soon I’d just recommend waiting a bit and restarting your PCs, then start up the Battle.net app again. It may sit on “Initializing…” for a while as it’s checking the WOW data files, so allow it to sit there and it should eventually move on to updating.