First of all, it is its touching plot and unique theme what caused my decision to recommend it to you.

解答：what为融合性关系代词(what=the thing that)，引导名词性关系分句，不可以使用外置主语结构。有以下2种改法：即把what引导主语从句提前到句首作句子真正主语，以及把what改为that与前面it is构成强调句型。

【1】what caused my decision to recommend it to you is its touching plot and unique theme.

备注：蓝色字体为主语从句作主语，后面为系表结构。
【2】It is its touching plot and unique theme that caused my decision to recommend it to you.

备注：蓝色字体为强调句型结构。强调原来句子的“主语”成分。去掉It is that为下面句子

Its touching plot and unique theme【主语】// caused【谓语动词】// my decision【宾语】 to recommend it to you【后置定语】.