So, you would never write "black," "AA" (even if your explanation is that this stands for Alcoholics Anonymous instead of African-American, remember that alcoholism is a disability), "Asian," "Hispanic," "crippled," etc. You'll never be able to satisfactorily explain why you made those notes. If the applicant has a name that is used by men and women, don’t write "female," because if you don't hire her, you'll be accused of giving yourself a reminder that this applicant was female. If the applicant is female, don’t write "appears to be pregnant" (pregnancy discrimination is a form of sex discrimination). If the applicant is obviously in his/her 60's, don't write "too old for job."
These ideas don't only apply to interview notes, but any other notes that a supervisor, manager, or HR employee might take -- whether in an investigation, disciplinary meeting, termination meeting, or any other setting that might bear on one's employment. An HR professional might write "discrimination" in her notes taken during a meeting terminating a 60 year old employee. That note could be an admission that the employee was being discriminated against, or an innocent explanation that because the employee is 60 years old the possibility of a claim of discrimination exists. If it's in writing, though, that employee will have to explain what it means during a deposition or to a jury. I guarantee that an employer does not want it left to a jury to decide which interpretation of that note is more believable.
The bottom line -- think before anything is committed to paper. If you have any doubt about whether something can be used against you, omit it. Education about the employment discrimination laws is key to this process. If you don't know what is illegal, it's impossible to know the traps to avoid.