Since I only have boys to scrap, a superfrilly rainbowed pink unicorn kit will do me no good and, hence, is not purchased. Since I used to have stepdaughters, I had NO IDEA how many supergurly kits there were out there until I went back to only boys. This is the biggest and easiest criterion to judge whether I will even consider buying a kit.
I will only buy kits with well done previews, nice logos, & NO misspelled words or other glaring errors. I'm a customer, and I expect professonalism and quality control in the scrapping world as I do at any store I frequent. Those designers who have branded themselves with a specific preview layout, font, etc. so that I can recognize their wares get my money because I know they take themselves and their business seriously and have spent some time and energy on making a name (ha!) for him or herself.
I am more apt to buy a 'questionable,' as in something I may not immediately be drawn to, kit if it is from a designer who I buy from frequently, anyways.
Although papers are not usually a draw for me since I only scrap with 1-2 papers per kit per layout, if they are very unique papers that have been handpainted, glittered, scrolled, etc., I will be more apt to buy.
Interesting and unique elements and a great multi-purpose alpha are my ultimate draws, however. Along the same lines, anything that mimics being a designer -- overlays, templates, etc. -- sucks me in, too.