Common/uncommon to have weak reference letters?

[Alastriss]

How common or uncommon is it for candidates to have weak reference letters? What aspect of a reference letter makes one a whole lot better than another?

[Jochi1543]

I think it would be rather uncommon, given how you can pick whoever you want (outside of family/friends) to write one. I know people in the US often have difficulties, especially if they went to large schools, because everyone wants 2 science letters, but in Canada there's so much flexibility in regards to who can write one, I doubt it's a common issue.

[Vlad]

Basically, the ref is good if they give you a good assessment and then write a ref letter that supports their assessment of you. In other words, they should elaborate on why they rated you well and how your activities/character makes you a good med candidate.