NOTE: I am talking about premades and cut-outs here, not ALL stock. And, not all premades and cut-outs are "bad." Just be careful!
These were really good suggestions, too, from amazing artists who truly deserve DDs. One of them has never had a DD before.
What am I ranting about now?
As an example, let's say you use an ivy psd file for your work. You need to know that the stock artist himself/herself took the picture of the ivy or that it comes from another legitimate source. Too many people pull images off of Google and cut them out to make "psd stock." So, if the stock artist doesn't say where they got the picture they used from, I would recommend not using it.
Likewise, a lot of people take stock pictures from sxc.hu or purchase them from dreamstime or shutterstock (or other similar places) and then make them into premade backgrounds. This is against the license agreement, which does not allow redistribution of the stock images. So, I cannot give you a DD for using these kinds of premade backgrounds.
EDIT: lenagold.ru specifically states that they get their images "from all over the Internet." That doesn't sound legitimate to me. Am I missing something?
These are not my rules!
Aside from not getting DDs, another reason for making sure you know where the stock you use is coming from is that, if dA finds out that the stock is from an illegal source, it can be scrapped or deleted from the site . . . AND if it is, your deviation that you spent HOURS making can also be removed.
I think that's my biggest reason for turning down DDs (aside from low quality or lack of clear concept): improper stock (followed closely by no credits, as you saw yesterday).
A lot of people are wondering why they don't have DDs, and there is often a very simple reason. The good news is that it can be easily remedied in the future. 
Hope that helps.
Jade
