I really agree with both what Eklypze and JJJ had to say. On one hand, I did think the finale was far too quick, on the other, I did like what they did in terms of showing that Dexter could connect with another person.
In terms of Deb, I actually thought that fit perfectly with what they established with her character in the fifth season. Early in the 5th we see a Deb who isn't bothered with killing a very bad person, and it was being established with her researching the barrel girls that she was really starting to feel some sympathy for what happened to them, and in a way starting to support the vigilantes. Deb really had a nice arc this season, and though I was annoyed that we still didn't get to see Deb finally find out who Dex is, I thought it fit with what was being established in her character.
And with Quinn, I though it was fairly obvious he stopped going after Dex because he was in love with Debra. That's why he lawyered up right when things started to look dodgy. He made the choice to leave Dexter alone because he cared too much about Deb.
But I still think my biggest problem was, after having this nice arc with Lumen, the finale came way too quick, and it was way too easy for them to set things pretty much back to square one. After all that character development with Lumen, she just leaves. Part of me would have liked to see her stay, and keep exploring how their relationship would progress, or something.
Also, as much as I liked the character development in this season, part of me is wondering if Dexter is getting a little too tame. He's definitely changed a lot from season one Dexter, but I'm just wondering if he's starting to get too heroic. I don't have a problem with it now to be honest though, but it's something that I've been thinking about.