Players to login once, and never log out for fear of being unable to even get in a queue.
This causes even higher server load, since you never kick them. Most of my guild logged in this morning before going to work and will stay on the server, preventing anyone else from playing all day. We're not the only ones.
Wastes players time as they have no choice but to hammer the login button for hours on end if they want any hope of being lucky enough to get in.

Implement a login queue. How could you possibly not have this implemented. Even with many MANY problems and deficiencies, this is by far the biggest kick in the nuts to your customers.
EVERYONE would rather see they are 50,000 in queue than to have to bash their head against their keyboard every 10 seconds to try to get in, only to have someone else get lucky and take their place.