I have long thought, and continue to think, that Luke's primary lessons from another person on how to use a lightsaber were his fights against Vader, and here it really shows.
As mentioned above, he's efficient and absolutely brutal. That is not at all a criticism, by the way. There's none of the flourishes of prequel Jedi. He's straightforward, and gets it done. He goes for the kill shot every time. He fights a lot like Vader- which makes sense. His primary teachers in the force were Ben (with about two days of instruction) and Yoda (old, doesn't have a lightsaber anymore).
But he had at least two fights with Vader, blade to blade. I can see a lot of Vader's techniques in how he fights here. Between ESB and ROTJ he learned a lot, and I'm fairly certain some of his technique was adapted from how Vader fought in ESB.
Even his use of the Force was efficient- using it to throw boxes at the dark troopers, or push them around, or crushing that one dark trooper. It reminds me of how Vader used the force to throw things at Luke in ESB.
In short, Luke fights like his father, because his father was the only real example of lightsaber combat he had.