I am going with what will make me do an impulse buy.
First color, the colors draw my eye and either lift me or bring me down.
Preview of the kit is really important, thee way a designer presents her kit either makes me have to have it, or pass it over.
I look at the shadowing on the elements as well, because how those elements shadow look is usually an indication of how easy or hard its going to be to do that myself.
If they dont shadow well, then my final layout isnt going to look great either.
I have in the past bought an entire kit just for one element.
Now I dont, I check out the cu stuff and get what I need from there.
The papers are also important, I really dont like bright and busy patterns, I find them really distracting in layouts.
So I look for some patterns and very subtle background papers.
Value for money is also important.
I also love the designers who sell the elements and papers separately. I find that is especially great for doing challenges and things at sites. And also because sometimes I just want a plain background but love the elements of the kit.

Mini kits are great if they have the right mix of elements in them. They are also great for challenges lol.

I have seen layouts in a gallery which has induced me to want to purchase the kit, clicked the link and the preview was so poorly done that it completely turned me off the kit, even though the layouts were what drew me to look at it in the first place.
I also love kits with splatters and things made with brushes.

These days I tend to buy more brushes than anything because you can do alot with them.

I also have my favorite designers and sites that I tend to purchase from. I like shop for all I want in the one store if possible.