I don't think the reading experience can get better, it's just going to get more distracting after this last years literation of readers. Especially on tablets like the Kindle Fire.
I like being able to look up words too, but I mostly like the easy to use search, the ability to go to an index and click the numbers to goto a page, and to find where certain things happen.
Plus the customization.
The danger that will come is when X-Ray gets so advanced that it just becomes a built-in SparkNotes/CliffNotes.