I think that the need for beginner / easy patterns is low -- there are a ton of free or low cost online tutorials and patterns to help the absolute newbie along, and any quilter with some experience and chutzpa can probably figure it out on their own -- so why buy a pattern.
I rarely buy patterns, but when I do, things that make it worthwhile for me
-- multiple sizes -- yardage breakdown for everything from wallhanging to king, and I also appreciate a pattern writer who tells you -- each block will require X-- as I like to be able to make my own adaptations.
-- a clever time saving technique or method that I would not have thought of before.
-- some input on the process -- best way to press seams, considerations in color and fabric choices.
-- online presence and support. An example of a pattern designer who does this well is Julie of Jaybird Quilts. She has Instagram, Flickr, blog, etc sites where people post photos of quilts using her patterns, and she actively engages with people who are making her quilts, offering tips and encouragement. I like this because over time you get a huge range of images that give you an idea of how the pattern looks in many different color or fabric choices.
-- I hear lots of friends say they only buy pdf or instant download patterns. They want to keep it on the computer, not cluttering up their rooms. You will probably find just as many people who say they only buy printed paper. So maybe offer both/either.
I tend to be more likely to buy a pattern for a bag or tote than a quilt -- just because my solo improv efforts there have not been so successful, and I think a pro has something to offer me in bag construction (sometimes I've been disappointed on that score).
A really good name for the design helps. Something that's descriptive, memorable, accessible. A good example-- "Chainlinked". Or "Gravity".
Good photography -- of the steps along the way (or if you are a wiz at line drawings, that's actually my preference.) And of the finished product.
I think it has to help if you're a bit of a prophet and can sense the next trends in home decor and design. Probably most of the pattern buying market is relatively young/new to quilting (the long timers tend to already have an accumulation of to-do patterns and idea) so you're probably trying to appeal to the younger aesthetic and their tastes. And their fabric interests. So, lot of people oohing and ahhhing over Cotton and Steel. They're fun and funky and fresh, and very on trend, but the fabrics are kind of different, and don't always lend themselves to every quilt pattern.
I think that patterns that are readily used with pre-cuts are always going to be popular. How many times have you seen someone post or heard someone in a quilt shop say -- I have this jelly roll but don't know what to do with it, or I love this charm pack, but I'm not sure how to make it work. I like patterns that specify fabric requirements as: x fat quarters, or a charm pack plus jelly roll, or one layer cake plus scraps equal to 1 yard. By doing that figuring for the consumer, you increase the chances of a google search leading them to your pattern when they type in "charm pack quilt pattern".