the logo and banner and thumbnails are definently a good first impression.
but also being enthusastisc is a good idea, sound like you enjoy what you are doing.
but also dont be quiet like a rock if you do letsplays, etc
something unique only that channel does, a funny person, someone whom can explain complicated things simple so even the most dumb people can understand things.
Some would say personality, others content, there are many reasons to like or not to like a channel. Mostly it is about what content you provide and in what terms do people enjoy to see you playing that.