Immersion is an essential element when writing a novel. This doesn't mean that it is the most important element (which is a topic that could start a world war) but in order to have enjoyable novel-length story it is neccassary. From a slush reader's standpoint, it is one of my criteria in order to pass a story up.
I don't want to make this post about all the reasons why I reject something because I feel like I've done that before. Instead, I want to talk about the ways how writers screw up plans and get me to read their entire story which is the only way I'll consider giving it a bump.