Despite being the leader, she's incredibly cold and unfriendly to many other members.

I'd say ill-mannered. Let's be honest, it's obvious that she favors some members more than others, and while the whole controversy over her posting pictures about her relationship seemed far-fetched in my opinion, I can't say the same for the picture she posted before Jessica's scandal. To me, she seems a bit unsuited to be the leader. (I can imagine another member taking her place as leader, but I choose not to.)

She is very moody and doesn't always put her profession face on

Finally! Altogether actually. After all these years I cannot forgive Taeyeon for treating Seohyun like shit and I have to say I despise even hate her for that. Really, how can a teammate show her disdain for another teammate in front of a camera? She insulted Seohyun on several occasions, e.g. during Intimate Note and Come To Play where she even had a nerve to laugh at Seo behind her back and it was captured by a camera. Like, REALLY?! She could have at least tried to act more friendly and understanding for the sake of SNSD. On top of that she's a LEADER for God's sake! A leader should be the most helpful and not be a destructive person. I haven't seen other members showing this kind of bad behaviour and attitude towards another member. Taeyeon is mean and just cold, bad. Ugh! I laugh when people say that she is a shy person. A shy person?! What the fuck! Seohyun is someone like that, not someone who detests shy people! She can have the most beautiful voice in K-Pop, be all cutesy and all, I still won't like her and won't forgive her. -_-
And also what is this all about "leader taking a good care of maknae"?! That's just a farce.

Agree with amysakurada and snsdlover4ever.

At least sometimes she could pretend she is fine, she's and idol she shouldn't make people think what's wrong with her

^ That was actually a good thing that she opened up. We can see that "idols" are normal people with problems, to say the least problems that are caused because of being a celebrity. Her confession actually made me really sad. Well, maybe she should just quit.

never like the way she favors some members...so unprofessional especially for a group leader

Moody, lack of integrity, unprofessional, I could seriously go on and on.