I filed a I-129F on behalf of my fiancee and got an e-mail message today with an update on my case status. They said that the sent out a letter requestion additional evidence or information. I was hoping someone here could help me settle my mind and let me know of some possible things that they could ask for. Is this a good/bad thing? Thanks!

You have to wait and see first what are they asking for. They probably going to ask you to provide more evidence about your relationship with your fiancee, but wait and see, it's nothing bad, they need something to process your application and you'll find out what it is when you get the letter.