This is a question that is interesting in more ways than one. One of the more common answers to this question i hear is naturally centered around performance, even though many times the person with the 'fear' of the performance hasn't actually tried to see what types of performance they could get? I would love to hear about specific areas where people have found the performance to be lacking, and the goals they're trying to accomplish when hitting these 'barriers'.
But above and beyond that, what other reasons would you have for not using Managed DirectX. Do you think the working set is too high? Do you not like the API design? Do you just wish that feature 'XYZ' was supported, or supported in a different way?
At the same time, what about the users who are using Managed DirectX currently. What do you like, and why?
You can consider this my highly unscientific survey on the current state of the Managed DirectX runtime. =)