One of the topics I see trivialized a lot in fanfiction is mental illness.
I think part of the problem comes from not actually understanding mental illness, but the fact there is still a stigma which goes with it. On the flip side as those with mental illness have tried to erase the stigma, I've also seen a trend of kids proclaiming on their profiles as if it were some kind of badge of honor that they are some kind of mental illness without actually getting diagnosed. Certain things, such as having two diagnoses which contradicted each other clued me into the fact the person was lying.
Of course, a few sides with giving the benefit of the doubt, yet the answer shouldn't be to let them treat their mental illness as an excuse but encourage them to actually get the help they need. Plus, I don't think we should give the benefit of the doubt to someone who thinks they suffer from depression simply because they feel sad. Everybody feels sad, so by that logic everyone suffers from depression when this isn't true.
Not doing one's research is a major issue which in turn typically results in not portraying the illness correctly. Another issue is the illness turns on and off conveniently when the writer wants, yet those suffering from mental illness – even with coping methods and medicine – can't turn it on and off like it is portrayed. There's also the fact the cause and effect doesn't add up.