I've been having the same issue. I am on a home network, unfortunately due to Verizon employee mistake we are routed through Kansas even though we live in TX. Needless to say my internet is excruciatingly slow.
I grew impatient with launcher.exe and snooped around.
1. Go to containing folder (wherever your wow.exe is)
2. There should be a Temp folder
3. Inside is WoW-4.3-5.0.15890-enUS-Downloader, open this directly
4. It will download for a few minutes, then a popup will say 'Could not connect to tracker!'
5. DONT TOUCH IT. If you watch, the download will continue anyway.
6. The download window will close itself, the popup will remain. LEAVE IT. Go smoke, whatever. The d/l still continues!
7. After a reasonable amount of time, click OK and open WoW-4.3-5.0.15890-enUS-Downloader again. Notice the amount d/l is still increasing.
8. Run Launcher.exe and enjoy
Sorry for the huge explanation, I'm not very techie. Figured it out by mistake, wanted to share a possible solution for anyone else.
thats from Hexius, an 85 draenei pally, the only alliance toon i have ever liked
also if you get the cannot run required agent try this fix