I think you can end up getting "enmeshed" even if the therapist is ethical and doing things right. It isn't even necessarily a bad thing as long as it is kept within the bounds of session, etc. and the T knows how to work with it.

Behaviors I'd worry about: blurring boundaries, love-bombing (overwhelming you with praise), negging (telling you you're awful and need them to fix you), acting more like a friend than a therapist (lots of personal disclosure, they talk about themselves more than you).