It's really a shorter answer to say what's NOT to love about Justin.
He's kinda perfect, but not in that way that makes you want smack him.
So let's go with why Brian loves Justin.
Because Justin doesn't ask Brian to be anything other than what he is, or you know, what he could be if he stopped pretending he doesn't care.
Because he's an adventurous little fuck, and not just in bed. He's willing to try new things.
Because he's not really a little fuck at all. He's a man and after season one, we're given absolutly no indication that Brian sees him as anything else.
Because he's a good man. Not just the best man he can be, the only man he can be, but one who fights for what he thinks is right damn the costs and damn what anyone, including Brian thinks.
Because he's fucking Beautiful. Have you seen him naked? Brian has. He's impressed.
Because he's comfortable in his own skin. Again, this is not just sexual. This is the intact ego thing. This is other people calling Justin a fucking queer, or not liking his work, or whatever and Justin not letting it shake him to his core. He grows, he evolves, he changes, but on his own terms, to his own tastes and when he loses his way... he finds the map and a better path. He doesn't need anyone to do it for him.
Because he knows when to let others do it for him, or help him do it, or whatever. He's not so pigheaded or strongwilled that he can't or won't ask for help when he needs it. There are times when he may get in a little deeper than he planned (pink posse, The Sap) but he always pulls back and then finds his center with the help of a loving partner and again, an intact ego, and an adopted family that supports him.
Because Justin lets Brian love him as much as he's able to and rarely pushes for more. After the arc of agony we never again really see Justin push Brian for emotional declarations. Justin says that he doesn't need them, and Brian takes him at his word, and thus...makes them, all the time, in actions, if not words. (although the words are there. ["by the way, i missed you"]) And finding someone who's willing to work with you and meet you at your level is a rare thing to find.
Because he's smart and funny and he's almost as much fun clothed as he is naked.
Because he's strong, really strong, strong enough not to push for what he's not going to get, and to leave when he's clearly not going to get what he wants (507) and to come back when Brian's willing to admit that maybe he was a assholic in regressing back to being 29.
Because they're OTP and it's Magical and it wouldn't be magical if all that other stuff weren't true.