I believe that to be a good YouTuber, you need to put off some different but creative kind of energy. You have to produce content that people enjoy seeing, and would like to continue to see. Some of it gets repetitive, but I enjoy watching OfflineTV and their League streams, because they are creative and put out good energy to the rest of the world when making their content. I truly believe that is what makes someone a successful YouTuber, and in turn, getting many subscribers, which continues to fuel the good energy.
I think it depends on what kind of content they are putting up. Channels showing a lot of promise are those which follow the current trends, and pay close attention to the viewers demands.
Also, doing giveaways and other contests help a lot in getting the number of subscribers up.