Here's something that thoroughly pisses me off, and with streaming MMOs every other month I encounter it all too often.
Variations of this problem occur with nearly every launcher I've had to use for the last few years. The less infuriating ones (such as Steam and Origin) automatically resume the downloads, so at least I'm not stuck babysitting them. The rougher ones (Webzen comes to mind) stall completely, requiring you to restart the entire launcher and in the case of some even rescanning the data each time before attempting to continue the download.
Other launchers in recent memory I've had this problem with include Glyph, Ignite and even the Secret World Legends standalone launcher. In fact, I'm having a hard time coming up with a title within the last year or so that didn't give me a hassle when downloading (at least anything over 1GB).
What really bothers me is that I have yet to figure out what's causing it. It's not my connection in general, as Speedtest shows no issues. Other internet activity (such as streaming music or video on the same or other devices) works just fine. Hell, I downloaded a torrent the other day (it was a legit file - those do exist) with no problems whatsoever, but attempting to get a game from its own launcher is a frustrating mess! As far as I can tell, my firewall settings are correct and my antivirus isn't throwing any flags.
Doing some searching, I've discovered this seems to be a problem others have as well, though it's relegated to just one launcher or title per post on a forum. Support responses either don't work or just go unresolved (or totally unanswered), meaning no conclusive results. I guess it's a mystery that will remain unsolved for some time to come.
If you're wondering, I'm currently on 48% downloaded. It would be a bit higher if I'd stop typing and just watch the damn thing incessantly, but that would drive me crazy. Still, I'm at the end of this rant for now and I have a lot of clicking still to do...