Disease!! Which is partly an organization/discipline problem - as others have stated, I get way too excited and pack too much stuff in, and often I'm watching tomato plants to see what is going to happen first - ripened tomatoes, or a fully diseased plant. Also, vine borers - every year I try to be really careful about them and it just gets worse and worse. The first year was kind of beginner's luck in that I didn't really know anything about them and I got tons of squash and didn't have much in the way of issues.
