I think therapists have an obligation to disclose anything that might realistically impact the therapy (e.g., blindness, an illness that might require frequent or prolonged absences).

I also believe they have an obligation to answer certain questions about their worldview, but only if a client asks (things like theoretical orientation, religion or meaning-making system, political leanings). I also think they should answer direct questions about things like sexual orientation, if they are partnered, if they have children, if they've had their own therapy, etc. I know there are therapists who disagree (and clients who don't want to know), but I would not work with a therapist who wasn't willing to disclose these things when asked.