For me, it would depend on how well I know the writer. If it's someone I don't know at all, but I feel strongly enough to comment, I'd give some positive feedback and add something general like 'there were (e.g., plot) points/issues that bothered me from a X (e.g., feminist) standpoint. I'd be happy to discuss those, if you're interested'. That way, I indicate the problem, but leave it to the writer to seek discussion. Some people don't want that kind of crit, while others crave it and long to have more contentful feed instead of the staple 'i loved it' or 'LOL'.