So when I try to log in, the verification process runs very quickly up to 100%, then it bounces back somewhere between 10 or 20%. Then it comes back up at 1.2 MB/s until it hits 100% again, and then it allows me onto the game.
But today when it bounced back, it didn't finish back up at 1.2 MB/s. It just froze at 90.7% and would go no further. This was mildly annoying.
Any ideas?

So today I tried to log in with one of my characters multiple times and got a critical error message every time. This is not the first time this has happened...a random character will suddenly just be log in poison and can't be played at all while the rest of my characters are just fine. I played retail for several years and I don't remember this happening, ever. I think there's a work around where I can use the "stuck/unstuck" function to help with this, but it's damned annoying.

@JMTv
I had the same issue for a long time, but finally fixed the problem. Try running your launcher as an admin. Right click on your launcher file and click 'run as administrator." This fixed the issue for me.
Good luck!