There's an often occurrence on forums where users say they will get back to a thread in a few days with some more information regarding the topic and they forget to do so. Usually it's something like this:
#1 Jason, 1st of March: "I am testing this equipment now. I will let you know if it's good in a few days"
#2 Paul, 1st of March: "Oh great, I'm looking forward to it!"
#3 Paul 15th of April: "Jason, did you get a chance to take a look at it?"
#4 Jason, 15th of April: "Oh yeah, I actually forgot about this thread. The equipment is great, I recommend it!"
My idea is that instead of posting #3, Paul could use a button next to post #1 that would send an email notification to Jason saying: "User Paul would like to remind you about your post #1 from 1st of March. Click here to visit the thread".