I will go for the last item in the list which is quality videos. The niche should also be interesting and be followed. If your channel is about pets and then you have a video about plants then that is way off the mark that your subscribers may be turned off. My personal choice to gain viewers is promoting the newly uploaded video in social media. Especially in Facebook where you can have as many as 5,000 friends and followers then that is already a big amount of audience.