I can think about any number of characteristics that would make them a bad therapist for anybody, not just for me (such as being judgmental, unethical, or dishonest), but to me it's more interesting to discuss those things that are really helpful to some people, but would be bad for me.

I think the top one of those is probably personal disclosures - I do not want a T to make them, I don't want to know if he is married or divorced or bi or straight or has five children and a troupe of performing seals, or is single and childless/child-free.... I just don't want to know, it's none of my business and could serve no other purpose for me than to make me uncomfortable. And yet, for some people it's quite hurtful when the T does not tell them anything about their private life.

(This is not at all related to being a blank slate, by the way. I would not want that in a T.)