Even though, IMO, I'm unusually resilient in the face of honest harsh crit, I'd say it's more peer-like to pair negatives with positives - point out flaws/things gone wrong, but at the same time commend for the good.
Criticism seems to have such negative connotations in most minds, generally. In the real world, it's often better to say nothing because the person in question either doesn't know how to interpret crit in order to extract useful information from it or it's not an appropriate topic to initiate in the first place. Plus, crit may not even be useful in the first place due to gaps in skill be too large, between crit'er and crit'ee.
But here in this site, that's the purpose. You're not here unless you want it. So not a whole lot of etiquette conscientiousness necessary. Just be human. Hold the crit recipient's best interests in mind as you attempt to help them understand how to improve. It's called "constructive" criticism, because there's also destructive criticism. Stick to the former.