Wasn't particularly impressed with this episode. It was never explained exactly what the 'Dark One' was, why he was feared and how he had magic powers. When Rumple stabbed DO, all he did as this uber-powerful guy was stab his foes with a dagger. Strange because in the beginning of the back story you see DO use magic. I thought that was the point, that he was a being of magic.
Also, earlier on in the season you see Rumple eviscerate a fairy, presumably with magic. So basically once you've seen this episode you don't get the impression that Rumple is a big threat, at least not as big of a threat as portrayed earlier on in the season. I also don't get the whole contract spiel that he kept on babbling on about throughout the season. We've seen instances of him not requiring a contract to get his way, and others where he is forced to agree to terms. It was never explained how that worked.
The thing is, if none of this was ever explained in his back story then it will never be explained ever.