[Ronsenshi]

What would you consider "going personal"?

If student is lazy or didn't think his work through - is it personal to call student on that - call him/her lazy? Tell him/her "you're not thinking!"?

Surprisingly - some people take personally critique of their work.

[Latty]

Telling someone they are lazy or they didn't think isn't critique of their work, it's a statement about them as a person.

If the work is wrong or bad, explain why. Insulting someone doesn't help them improve.

If you argument is people won't change unless they are made to feel bad, people said the same thing about physical pain - banning corporal punishment would mean no child would learn again. That was nonsense, and so is the idea that making people feel bad is necessary for them to improve.