[KyleGoetz]

To build on what Sashimister has said, question words are never followed by は. They are only followed by が when marking it as the topic.

Think about it this way: は is a topic, not subject marker. A question word can never be the "topic" of conversation, right? I mean, it's a nonexistent concept.

It can, however, be the subject of a sentence. Hence why が is OK but not は.

To explain another way, a way of translating Xは is "As for X." So for なにはおもしろいですか。 you would have a translation of As for what, it is interesting?" This makes no sense in English! Let this help you remember that it similarly sounds wrong in Japanese.

The correct way to put this would be なにがおもしろいですか。What is interesting?