There are so many fantastic elements to this episode.
First the quest itself. A journey to the fisher king to take his trident, going through the perilous lands; it doesn't get much more Arthurian. Then you have all the great characters like the guy at the bridge, who knows way more than he's telling. His character totally takes the episode up a notch, him and the fisher king of course. The fisher king offers Merlin a warning to the future that we all know is coming, its great that a long term story arc is being referenced here.
Then you have the return of GAWAIN!!! Who apparently acts as strength while Arthur is courage and Merlin is magic; obvious but i still like it. Not to mention the relationship of Gawain and Merlin, somehow it feels as though it will rival that of arthur and merlin.
Then you have lovely Gwen. Finally! She sees Morgana (who is such an awesome character) in action and doubts begin to set in. Nice that the future queen is even more in arthur and merlin's corner now. There's romance, action, adventure, and comedy. This could have been a two hour episode if they had wanted to. This one really sets the bar, i can't wait to see what's next!