There must, MUST!, be a clear pop-up warning when a city queue becomes empty. I'm a relatively hardcore gamer and I miss the zzz signs often in the kingdom overview on the left. Imagine how many casual gamers will do the same mistake, even if you personally don't have this particular problem.
I know that sometimes you want to leave the queue empty. When you don't want to build anything, there should be an ignore button when you get the "no queue" warning. But the game needs to stop you and inform you that a city has stopped building when it happens! PLEASE!