On Instructables, contests have tended to had very limited specifications beyond a general theme they work to. I propose this trend be broken in favor of something a bit more actively competitive: hold contests where the objective is set and have people compete to meet that stated goal as set by the contest host. The parameters that entries would be judged upon would be laid out in the briefing, as would any material or cost limitations.
Holding competitions in this manner would entice people to competitively build something to meet the requirement the host sets and to do their best to hone their project to succeed in the relevant parameters.
I think this would be a nice change from the chaotic randomness of current contests where there isn't much competition among entries, besides wooing people for votes to win finalist positions.