Color always grabs me first, and a well-designed preview is nice, especially when a designer includes a preview of the papers and even paper closeups that show 5 papers to a preview.
I also like to check out the elements. A kit needs to have embellishments to warm up or decorate the page, and items like flowers, greenery, brads, pins, staples, stitching, borders, frames, journal spots, themed knickknacks, and wordart are great. Hmm, I guess I'm saying that I like my embellies to have a purpose as well as look good, not be merely decorative.
I'm definitely into journaling, too, so I like elements that help get the journaling done and looking good in the process. To me, the story is what is important, the who, what, when, where, how, etc.
Most importantly, when I'm choosing a kit for purchase (or even a freebie to download), I look closely at the originality and quality of the items. If the designer relies heavily on CU items, I don't give the kit even 10 seconds of my time.
Told you I like to journal ... you should see how much I left out!
