It is worth discussing the question.为什么不对?

不对
正确的是The question is worth discussing.
sth be worth doing

改成
It is worthwhile discussing the question

具体请看
worth 的用法
worth,worthy,worth-while都为adj.意为"值得".
1.worth：be worth + n.当名词为金钱时,表示"…… 值得……"
be worth doing sth."……某事值得被做"
The question is not worth discussing again and again.
2.worthy：be worthy of +n.当名词为抽象名词时表示"……值得……"
be worthy to be done "某事值得被做"
The question is not worthy to be discussed againand again.
3.worth-while：be worth-while to do sth "值得做某事"
worth while：It is worth while doing sth
It is worth while sb to do sth.

还是说the question is worth discussing 比较好。
因为像你所说的话，it是形式主语，也就是
discussing the quesion is worth显然是不合适的。因为worth 这个词意为“值……的”，所以不会单独使用。一般后面可以真接加钱数或是doing