It demands your attention and respect. So of course does a good novel or a poem. But here's another difference. You can't do it all by yourself. Theatre is by its very nature a communal event, so even when you think you are simply a spectator, in actuality you are a participant in the crowd.
Theater is a performing art, designed for a live audience.
When it's great it becomes part of you forever, a part of your intellectual and spiritual baggage. There's also another major difference. Unlike any other genre, theatre is collaborative, not only in the process of creation but in the very execution. All my scripts have gone through this process. In the theater you are never working alone.