I think of myself as kind of an overglorified hack who isn't very good at writing and everyone just thinks I am because I kind of have that sort of air about me, to be honest... it's difficult to categorize what my strengths would be. Especially since I do so little of the actual practical writing myself. But I will try to do this anyway.
World-building detail, character motivation, villains, and clarity.
When it comes to world-building concepts of how societies or people work, I'm great at that. I'm not so good at making the actual physical world itself, but cultures are my specialty. Explaining in detail what sorts of technologies they use, how they think, why they think like that, how organizations within these cultures work... that's my thing. I do that more than actually write the story itself because I find it so fun. Character backstories, motivations, and psychologies tie into that too. I can make some really good ones.
Villains tie into that. I am a very paranoid person, and I can take the point of view of a villain very easily. The villain must constantly be on the defensive from the hero, and I feel like I'm always on the defensive from the world. So I tend to make very intelligent villains who don't fall into the usual trap, and seem less like clearcut evil... and more like just people. It's easy for me to sympathize with them and give them ridiculous details that make them a huge challenge for the heroes to overcome. And because of my strength of making character motivations, the villains will usually have some good reasons for what they're doing.
Related to that, I am also pretty good at plugging up plot holes or flaws in logic. I try to craft a story that has very few of those, thinking of all the possibilities and scenarios.
When I write, I am usually very clear about what is going on. I hate ambiguity and loads of flowery prose. So I tend to get to the point, and make my word choices vaguely poetic enough not to be ridiculously terse, but not so much to the point that it's incomprehensible.
I only work in one genre - sci-fi/fantasy mixtures with JRPG-like adventure stories that mix drama and comedy. Not really interested in anything else.