Was this at a hub or an outstation? That makes a HUGE difference IMHO.

If you want a faster response, call Consumer Affairs as opposed to writing the letter. Then again, poopie happens and this look like just one of those occurrences. I doubt you'll see much happen. I'm sure US could have handled this better, but I don't see you getting much compensation for this.

Also, this doesn't warrant a letter to DOT. You weren't put in danger, just inconvenienced a couple of hours.

Look at it this way, about 10 years ago, I had two flights out of MSP on US that had mechanicals due to the PA system not working. One flight was canceled (the first one), the second went off with the FAs using the bull horns to do announcements. That was a reason to complain as US was inconsistent in the way they handled things.