Speak with Lady Jaina Proudmoore, then break into the lower city and secure it for the Alliance.
The time has come. Our forces are well-enough entrenched here that we can risk pressing forward, into the Thunder King's lair. Unfortunately, it doesn't look like he'll be opening that gate anytime soon.
I suppose we'll have to make a gate of our own then. Let me know when you're ready, <name>.