I think Cheer's right to say that Natalie is engaging, but I think her engagement with her audience goes beyond just getting them involved during her porno music/comment time. Personally I watch Natalie because I feel her WHOLE video engages me. She talks about events that happens so regularly to me, (I would say everybody, but hey, I could be the other awkward animal in the world) and makes a hilarious video out of it.
Furthermore, Natalie is famous for her monologs. Something I truly enjoy as well. It breaks up the boring presentation type Vlogs alot of people do. Other YouTubers employ different techniques such as random humor, but I feel Natalie's monologs gives her versatility in delivery.
Making YouTube videos have become somewhat of an art/science. There are things the general mass look out for. So much so that there has been a game show on YouTube itself looking for the next Internet Icon, and there are judges to determine the success of the videos.Judges Ryan Higa (Nigahiga) and Christine Lakin along with various guest judges (i.e jennamarbles, kassem G) comment regularly on engaging the audience, and delivering the message it seems like if any of them wrote a journal article citing the top 5 things to do in must do in a YouTube video, it would definitely make the list.