I'm a sucker for covers that are type based AND have a clever or witty or visually appealing concept behind them too. Like these.
And then there's white space. I think type should be able to breathe generally speaking, as shown above. That being said I also really love this book cover for one of my favorite author's books which is the complete opposite of the whole white space thing. But it is still a very clean and simple design. The title is Daring Greatly and the type is in this colorful but subtle gradient that stands out visibly against a gray background. And the typeface is also HUGE on the cover. Everything about the title is daringly bold. The book itself is about sort of daring to put yourself out there. It's a great read! And the cover is, in my opinion, beautifully done as well.
I love that so much can be said with type. Type conveys a lot about the feelings or thoughts behind the author's intentions or what the characters in a story go through. The psychology of it is fascinating!