It's been years since I've seen "Unnatural Selection", and I must say I like it much better than I remember. It's the first episode of TNG to treat medical science and precautions fairly seriously - when they release something has gone wrong on the Lantree they try to determine what
rather than beaming over unprotected and poking at the problem with their bare hands ("The Naked Now" solution), and Pulaski's efforts to find a cure are dramatised with their problems being pretty easy to follow - as opposed to the first season's tendency to have a flustered Doctor Crusher come up with a cuire at the final hour because... science?
I'm not saying it's a great episode or anything, but it's a competently executed sci-fi story - something TNG would do so often, but which it's done fairly rarely this early into its run.
It's also the first episode to refer to O'Brien by name, and the first one where he plays a significant role in the story.