In the virtual classroom nearly all communication is written, so it is critical that students feel comfortable expressing themselves in writing. (Para.5)
Meaning: In the online classroom, people communicate with each other by writing, so it is very important that students do feel at ease in writing to express their opinions.
critical: a. 1) very important; very serious or dangerous
I didn't realize the unemployment problem was all that critical here. 我原来不知道这儿的失业问题这么严重。
Note: In the that-clause of "it is/was critical that ..." pattern, we should either use "should + bare infinitive" or simply use "bare infinitive." Very often, fluent native speakers use only the 'bare infinitive.'
It was critical that the teacher (should) not punish the students who gave incorrect answers. 关键的是那位老师不应该惩罚回答错误的学生。
It is critical that you (should) keep up with the progress of an online course. 跟上网络课程的进度十分重要。
2) finding fault
Don't be critical of him; he is a beginner. 不要对他挑剔, 他是初学者。
She looks on everything with a critical eye. 她用批判的眼光观察每一件事。