I am definitely not an expert when it comes to this but here we go...
No.1
Audio
I for one think that audio is the most important part of the video. I think that clear speaking (that's loud but not so loud that I have burst eardrums!

) is very encouraging and there should be no - to very little - background noises. Especially TV. I get really distracted when I am trying to listen to what the person has to say but sponge bobs on in the background (not hating on sponge bob of course! ). It's very annoying for the viewer to try and listen and most of the time you will get distracted and lose your thought bubble.
No.2
Quality
Of course I'm not saying you have to have an expensive camera to do great videos on YouTube. This is mainly focused on editing and lighting. For me editing is a very big part of a good video. If you have a fun intro and can cut out unwanted clips then you have yourself a decent video (to make it good or better read no.3) I get discouraged to watch a video that is completely raw. Also if you decide to make a video then make sure you have a good filming spot. Lighting is the main thing for me considering quality and if you are filming in the middle of the night - chances are I won't be able to see a thing!
No 3.
Content
Of course without good content a video is just not a video that makes much sense. Of course make whatever type of videos you will never tire of and always be yourself but remember that your video has to be worth watching. If it's informational and has all the right facts - you will be gaining a lot of views!