Mostly the similarities in vocabulary - it's exciting to be able to reconstruct what could may been spoken thousands of years ago by noticing the links between present-day languages! But I also like the case system, mostly because it feels like English has lost something of its poetic utility by dispensing with it. To expand on that, with cases, you can allude to the relationship between a thing and a situation without actually specifying what that relationship is, leaving something up to the reader's imagination.