It’s displaying the first paragraph, which is exactly as intended. I’ve probably just phrased the setting poorly (in all plain-text editors, one paragraph = one line).
The “show first line …” feature is not really intended to show your review or even a small snippet of your review – it’s intended to let you write something specifically for the widget. The by far easiest way I could think of doing that was to show the first paragraph from the Goodreads review. That way, you could reserve that paragraph for what was to appear on your blog. I of course realize this is not ideal, since it puts some restrictions on how to write your Goodreads review, but other solutions not based on data from Goodreads (e.g. a local table in your WordPress database) would be significantly more work to implement and was out of the question.
Thanks for the suggestion to implement the “first X characters” feature, but that won’t be added. Simply put, it would be far from trivial to make such a feature, because that the Goodreads review can contain HTML tags which should definitely not be broken.
Sorry, and I hope you can still make use of the plugin. ??