writing grad school reference letters
For graduate school admissions, the goal of the reference letter is to recommend a candidate who you think will make an excellent future researcher. So you need to explain what the candidate has done for you, and why you think that gives the candidate the preparation she needs to succeed as a graduate student.

That means you should talk about:


The skill set the student has developed under your supervision.
Any noteworthy accomplishments (papers, awards, etc.)
Anything else you believe highlights the candidate's independence as a researcher, leadership skills, potential as a teacher and researcher, and so on.


Since your audience is fellow researchers in your field, effusive praise for someone who doesn't merit it is not helpful. I've known professional working relationships that soured because one PI tried to "sneak" a less-than-meritorious candidate through into a colleague's group as a postdoc.