I think they were so focused on making Dexter so in love with her it felt forced and unbelievable. There are aspects to Hannah's character that could have been interesting but they don't really follow through on any of it.

Something I thought was actually really interesting is that ok in s7 Dexter turns to Hannah because he's feeling rejected by Deb. He feels like Deb will never understand him but that Hannah being a killer will. But Hannah isn't a compulsive killer she's a practical killer. She doesn't understand him at all. This is a point of contention in ep10. She tells him the dark passenger isn't real and he just ends up agreeing with her by the end of the episode. And of course this has no real lasting effect he goes right back to believing in the dark passenger. It's such a heavy-handed lackluster episode that fails to really explore the way killing is part of an obsessive-compulsive cycle for Dexter.

I'm not sure why they didn't let conflict like this fester especially because they must have known Hannah was going to try and kill Deb and Dex was going to turn her in for murder by the end of the season.

They were trying so hard to make them special but it fell flat. When Hannah was kidnapped and Dexter confessed to her he never felt that kind of fear since his mother died in front of him, I’m sorry what? Wasn’t even that long ago that his baby son was kidnapped and almost murdered by a religious nutjob but ok. 

Another thing that didn’t make sense to me is that Hannah is a serial killer who has killed people, including innocent people. They wrote her that way presumably so she could understand and accept Dexter. Yet she is also supposed to be one whose love can “save” him? And she does not need to kill so presumably can stop anytime if Dexter stops. It just looked like they tried so hard to make her the perfect girl for him which doesn’t make for a very compelling dynamic. People argue she made him question the “Dark Passenger” but first as you say they don’t do much with that but also it’s not like it wasn’t questioned before. Literally Lila did this in season 2 and Hannah often feels like a knock off that we are supposed to actually root for (Love Lila on my end, great character). 

And of course the two unforgivable sins that are that Tumblr fanfic sex scene and making Dexter forgive her for trying to kill Deb while also conveniently later making Deb be fine with her brother dating someone who tried to kill her, after half the season was dedicated to them finding their way back to each other.