When worlds are full of players (more than 950 players online), free account players have to wait until a space for a new player is available to be able to login, premium players can still login without having to wait any queue. However, when a world becomes really crowded (such as in Public Test Servers), even Premium Account players have to wait in the queue; but, surely, they "walk" a lot faster than the Free Accounts.
Before waiting list was included in game, free players had to try to login when there were more than 950 players online, this could be a hard task on a full world and is very unfair as its mostly chance and connection speed that decides which player will be able to log in.
Every place you will have to wait 2 seconds more, starting with 5 seconds if you are 1st, so if you are in place 60 you will have to wait 123 seconds (2 minutes and 3 seconds). The formula is, where x is your place:
$ ((x-1)*2)+5 $