Let's see... hmmm.
For me, my major weakness is character development. I have the most difficulty with grabbing a personality and making it seem perfectly real. I have a nasty tendency to tell what a character is like, rather than showing it. Characters are difficult on every level for me.
I know my strengths quite acutely, though. Plot is my biggest strength. These elements come to me early in the creative process, and though they often change during the evolution of a given story, I do have a knack for plot elements. Of course, since you haven't read my novel completely, maybe I just think I have a knack for plot and I really don't.
Another one of my strengths are my magic systems. They are extremely rule-based, and I apply rigor to them on as many levels as I can conceive. My magic systems are very developed in this manner. If I can't think of a solution to a particular problem, I will stress about it for a long while. When I figure out the answer, it is usually fantastic. Also, I always make sure that the plot and setting (and characters, too) are integrated with the magic system. Usually, the magic system precedes any other element.
My greatest weakness besides character development is revision-itis. I have never gotten past 30,000 words in my novel. Around that point, I realize that I've done some terrible error on some level (the most recent was character-related) and I needed to rewrite so I got the sequence of events correctly. I've never truly left what I considered the beginning.