I find her entire character one dimensional and absolutely irritating. The only sliver of character development she's had is when she walked away from Dexter and Lumen after they killed Jordan Chase. She's constantly angst ridden about her past/her family's past and is forever making bad relationship choices. She's extremely annoying.
Most of the supporting cast are highly irritating, actually. Dexter appears to be a show which is centered on the development of a singular character, with a performance by the main character that is absolutely exceptional, yet it is at the behest of the other characters; everyone else is underdeveloped at the expense of Dexter himself. Which was ace to begin with, but by the time the fifth season rolled around I wanted more.
And the fifth season left the entire motive of the antagonists unanswered, too. We get that the people who were killed deserved it, but we never find out why they did what they did Miami Metro Homicide included.
It's a show which has become increasingly ridiculous as it has progressed. Which is a shame, because I had a lot of love for it at the start.
I've yet to see season six, but I predict that Deb will become a Sergeant, with Batista becoming Lieutenant and LaGuerta becoming Captain - that's how predictable it's become.
EDIT: I should say that I've just watched five seasons back to back, and I think that by doing so it's made all the flaws very apparent.