What (to you) makes a successful YouTuber?

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Like many people, over the years I turned to YouTube for various purposes from listening to music (including exposure to new artists) to How To's to the user just vlogging or plain being silly for the camera. Sometimes I am stunned at a particular YouTuber's popularity - either because they're unprofessional (by my standards), because they barely do anything in their videos, or because what they do in their videos isn't anything that your average Joe or Jane couldn't pull off every bit as well. (And I must admit sometimes it's because they make me mad by doing covers of my favorite songs and all their over-zealous fans gush over how their version is better than the original when it's NOT, but that's a different story.) Other times I am stunned because the YouTuber may display some pretty phenomenal talent, ideas, and other innovations to their videos and barely get any hits at all. I do realize a lot of it has to do with promotion, but sometimes even the laws of marketing are defied when people who never promote themselves get seemingly nonstop adulation whereas people who do more to promote themselves are chronically overlooked. Personal taste, of course, also plays a role, so my question is: what has to happen in order for you to be interested in someone's YouTube activity? What do you look for in terms of content and delivery? And what is your favorite subject on YouTube - do you like to be entertained by music and the like, do you like vlogs, or are you more the DIY/How To type, etc?