I have a lot of people I contact who tend to be slow in contacting me back. So sometimes they need some prodding to give me the info I've requested. That makes this search really helpful:

list:"Waiting On" not completedWithin:"3 days ago"

So I email or call them, and add a task to my Waiting On list. 3 days later, it pops up in my daily to dos (using my ridiculous search) and voila, prod prod and I get the information!