He sings amazingly. He is super attractive. He's a tremendously talented actor.
And he's a sweet guy too:
That last Glee episode... it set up so much conflict for the future. And so much good stuff. Puck the bad boy is stricken with adoration for baby Beth, and is willing to change his entire existence just to be able to be a dad to her. Holy. Cute. And Mark Salling did it SO well.