The difficulty of a text classification task depends on various factors, but typically, the task can be difficult if (1) the amount of labeled data available for learning the task is small; (2) it involves multiple classes; (3) it involves multilabel categorization, where more than one label can be assigned to each document; (4) the class distributions are skewed, with some categories significantly outnumbering the others; and (5) the documents belong to the same domain (e.g., movie review classification).
when the documents to be classified are from the same domain, they tend to be more similar to each other with respect to word usage, thus making the classes less easily separable.