For me it is Thomas’ lack of questions during oral arguments. A Justice’s questions tend to telegraph their inclinations, giving the impression that Scalia arrives at the shared conclusion first.
I think some people bring in the issue of race where it doesn’t really exist. Liberals who dislike Thomas for being a conservative have a legitimate issue - Thomas is a conservative. It’s no different than conservatives who dislike Ginsburg for being a liberal. But I don’t feel there are very many people who genuinely dislike Thomas for being black or Ginsburg for being Jewish. Accusations that the dislike of these justices is based on race or religion is usually an attempt to discredit opposition to them as being irrational.
I think you are wrong, unfortunately. I think there is a strong view in certain parts of the Liberal community that Thomas is particularly dislikeable because he is a black conservative, and is somehow selling out his community.