The bad: The origins of Daenerys's relationship with Khal Drogo are disturbing; she is essentially sold into marriage with him, leading to sexual trauma and several heartbreaking weeks where Dany feels like a piece of property. (Though it's worth noting that in the books, the sexual relationship between the two is far less problematic.) The khal is gruff, has no time for delicacy, and solves his problems physically. He is also extremely stubborn, which eventually leads to his death.
The good: Famed warrior Drogo can handle everything that a strong woman like Daenerys has to offer and then some. Remember the look in his eyes when Dany takes charge in bed and wants to "look upon his face"? He loves her strength and celebrates her for it.
As their relationship develops, the khal empowers Daenerys to take charge in the khalasar, backing her up when she is questioned by men in their tribe. When he discovers that she is being abused, he doesn't stand for it: he kills Viserys, and brutally so, for daring to lay a hand on his khaleesi.
Drogo teaches Dany about strong leadership and immerses her in the dothraki culture. If it weren't for him, she would not be surrounded by fierce fighters who fight by her side through all seven seasons of the series. He's loyal, too — when Daenerys meets with Drogo and their stillborn son in the House of the Undying, her husband tells her that he will not ride into the Night Lands without her by his side. Even in death, Drogo doesn't want to live without his queen.
It's also worth noting that Drogo is Dany's first true love, and her relationship with him shapes nearly everything about her life journey. He also gives incredible gifts (that silver horse!) and thinks up the best nicknames ("Moon of My Life"!), which are huge relationship perks.