Sometimes you're looking for an animal which does NOT have a certain trait. Let's say you'd like to add a male Clown to your Ball Python collection. You like the Spider gene, but some of your females are Spiders, and you heard that the homozygous form is lethal. Before now you had to manually ignore those search results. Now, the "Excluded Traits" field in Advanced Search can remove Spiders from your results.
Try it out: Currently there are 539 Clown Ball Pythons for sale, 49 Clown Spiders, and 490 Clowns without Spider.
You can also exclude traits in Basic Search (the plain text box) by using a minus sign directly before the trait name (e.g., "-Spider"). If it's a multi-word trait, you'll need to use underscores (e.g., "-Bumble_Bee")
This trick can help for another situation as well. A incomplete/co dominant search will include both normal and super forms, so "Pastel" will also return "Super Pastel" (not so for recessives). If for some reason you wanted to exclude Super Pastels, you could search for "Pastel -Super_Pastel".
What trait are you actively avoiding, for what species, and why?