These are always thought-provoking threads.
For me, I think one of my biggest weaknesses is just a lack of creativity. Not a total lack; I mean, anyone who comes up with an idea and writes an original story has some level of creativity. But I'm just not great at coming up with those really unique, outside-the-box kind of ideas. And on the rare occasion that I do come up with something that seems out of the box, I immediately think, "Psh, how am I going to write that?" So I usually take more basic, realistic ideas and try to take them to a new level and write them the best I can.
Another weakness is similar to Kelly's first one - I feel like sometimes my writing has that lack of balance between description, dialogue, and action too. When I was a beginning writer, my stories were mostly dialogue with some basic action and not a whole lot of description or detail. As I grew as a writer, I realized that and tried to work on being more descriptive, and I think I have gotten a lot better with that. But now, sometimes I'm too wordy with my descriptions, and I have narrative parts that just go on and on with no dialogue and not much action, and I wonder if those parts kind of bog down the flow of my stories.
Strengths... one of them is definitely grammar. Aside from the occasional typo, you won't find mistakes with spelling, grammar, or mechanics in my stories. I learned early on that good writing in the technical sense makes up for a lack of good content LOL. I BSed my way through many a paper in school and got an A, not because of what I actually wrote, but because of the way in which I wrote it. Flowery writing and proper punctuation will get you far.
Another strength that I've honed over the last few years is that I do my research and try to make my stories seem as realistic as possible. I look up everything I don't know about or am not sure about, from places to medical conditions, so that I can get things "right." I've become kind of an obsessive perfectionist about it.
Last, I think I'm pretty good with dialogue. I try to write lines that I can imagine people actually saying, the way they would say them. I think my dialogue sounds natural, not stiff and robotic, even if it's a little more eloquent than I can usually sound in real life LOL.