The key to any successful YouTube channel—the key to building an audience anywhere, really—is consistency.
Not only do you need a recurring theme across your channel’s content (like the format, the subject matter, or the niche you’re speaking to), you also need to communicate what that is to viewers. You need to build a brand for your YouTube channel.
Viewers don’t have time to figure you out and what you’re about. And when you think about it, people don't subscribe because of the video they just watched, but because of the expectation of more content like it in the future.
Even massive YouTubers that don’t seem to have a specific “thing” like Pewdiepie and Casey Neistat, actually did have a consistent brand of video content before they got to the point where they could branch out and thrive on their personality alone.
So, especially when you’re starting out, figure out what the premise of your channel is—what your promise is—and communicate it at a glance.