Ahhh....so many things to say about channel banners.
A lot of new Gaming Channels in particular tend to clutter their banners with all their favorite videogame characters and it becomes very hard to look at. When I think of engaging banners these things come to my mind; Values, Texture, Legible Typography (Text), Engaging, Rule of Thirds, Composition, Colors, Foreground and background images go well together, and so on.
Now channel banners don't have to be complex. If it looks good it looks good and you'd know it (I'd hope). Just keep it simple and have it match the theme of your channel and it will be just fine. =] But it can't be boring either of course.