There are a lot of considerations in designing earrings once you get past the phase of just making a dangle and putting it on a fishhook finding.
Length: I'm tall so I tend to accidentally make shoulder-duster earrings when really I was just having fun using up a lot of beads. So make sure your earrings don't get too carried away with fringe & baubles if they will be worn by someone who's not an Amazon woman like me
Weight: Similarly, most people don't want their earrings to be too heavy so always be aware of this. Frequently wear your own creations to test the weight of especially large earrings. Ways to lighten earrings other than making them smaller: use bone, pumice, sequins, feathers, leather, or plastic beads in the design. Try to find thinner metal findings that weigh less. You can also make heavier earrings with clip-on findings since they will distort the ear less. Check the pages of recent Vogue magazines and you'll see catastrophically large earrings being held near the ears by metal headbands (not attached to the ears at all!)
Balance: Sometimes you put parts onto a chandelier finding and it doesn't hang straight, or the suspension chains for the drop are not symmetrical. Be sure to take time to make sure all the parts are suspended nicely & symmetrically.
Attractiveness: Obviously we all wear jewelry to look better so be sure your creations will enhance the beauty of the wearer. This is certainly a subjective thing since some people wear the same pair of earrings every day and others always want a new, unique pair as I do. But choose your audience, which is generally people similar to yourself, and respect that aesthetic in your work.
Versatile: The best earrings go with everything. The closer you get to making styles that can be worn by anyone, anytime, anywhere, the better at making earrings you should consider yourself to be. Neutrals such as wood, metal, leather, and shells can be used to make endless combinations of earrings that will go with jeans, skirts, dresses, and business suits.
Let me know in the comments what earrings you're making or what your favorite styles are,
--Pam