"The content is good, but it is too short."
Translation:התוכן טוב, אבל הוא קצר מדי.
I think התוכן טוב, אבל זה קצר מדי should be accepted, but it means a slightly different thing. Bare in mind there is no neuter in Hebrew. When referring back to a noun, we don't use זה as a translation for "it", we use either הוא or היא depending on the grammatical gender of the noun, in this case תוכן is male and therefore הוא קצר מדי. If you say זה קצר מדי you mean: the whole thing, the general something. If I asked you: how is the presentation that I made? You could say: התוכן טוב. אבל זה קצר מדי! Two unconnected comments, where "זה" doesn't refer to the content, but to "this general thing of a presentation". I hope this explains it... :-P