Consistency - all the best channels have a unique style and a specific language they talk to their followers in.
Regularity - They all have a timetable and regularly post. We're talking several times a week. This is how they hook their subscribers.
Leadership - When you are passionate about something, people will follow. They often have a leader who imbues the qualities that their subscribers admire.
Creed - They have a specific purpose, a belief system, a philosophy which runs throughout their content.
Specific - They are niche. They are the best answer to people's questions.
Communicate - They listen to their audience. They create content based on feedback. They invite conversation and debate.
Some great examples: Jenna Marbles, Michelle Phan, Sorted Food, Jack's Gap, Community Channel.