A lot goes into making a game, and each game and designer has a different mix of focus and needs for their games. Off the top of my head we have: world building, rules creation, technical and prose writing, art, layout and visual design, feedback and iteration, promotion and marketing, and community support skills.
Collaboration and contracting are good ways to make up for weaknesses, but so is practice and patience.
So what are you best at? What are you working on improving?
I am pretty good at creating a compelling plotline for my players to follow. I think the heart of a good driving conflict is proper motivation. I make antagonists that have clear motivations that fit the plot and world we are playing in.
I am currently trying to get better at drawing so that I can give my worlds a little more flavor than just a bunch of descriptive text.