Every. Single. Thing.
Your personality is to highlight your style, and go along with your branding. Starting out, a lot of people decide to not "brand" because they see it as selling out; which is silly. The entire point is to have a theme, and roll with it. Doing so doesn't mean you've lost your way, and now all you care about is money. People associate branding with money, and which just isn't the case. In the YouTube universe it means your style, something that makes your channel unique, and could even be something that connects you with your audience.
Brand your thumbnails, how you title your videos, have a few funny lines to set you apart in your description if you'd like, etc. etc. All these things can help you in carving out your own slice of a YouTube following.