Make comparisons to illustrate their success. To put the candidate’s accomplishments into perspective, include comparisons so that the recipient will have some basis to understand why you are recommending this person.
Do graduate schools care if letters are confidential or not? In general, graduate programs prefer confidential letters. Admissions officials say that it displays more confidence on the part of the applicant if letters are "confidential" (meaning you, the applicant cannot see the letter). The Letter Service is required to indicate on the letter if it is confidential or non-confidential.