That was shit. The whole season. Goddammit.
they killed Doakes off way too early in the series' life and it suffered for it. He added some danger and gravitas that the series never got back, certainly not from Quinn. If you look at it, Dexter only really worked due to the circumstances surrounding him. It worked with Doakes adding tension, with Deb protecting him on the police end, and with Rita protecting him on the home end. When they offed Doakes it went into a place where they introduced Jimmy Smits and that "main baddy" in each season. For the most part other than the let-down finale the season with Smits worked. Of course John Lithgow's brilliance is really what made season 4 shine as well. However the cracks at that point were showing. Without Doakes around any and all sub-plots involving Miami Metro Homicide dragged the show down. They started focusing on shit no one cared about to fill time. It worked for season 3 and 4 because it was almost like a tease because you wanted to see what would happen next with the main baddy because they were so strong, and the time spent with Rita and her kids was worth it because it offered a good juxtaposition with his murderous ways. Getting through the other crappy stuff was worth it to see the rooftop scene with Prado, or him going to Trinity's home for the first time. Then they offed Rita, and suddenly it all began to unravel. Again one of the bench-mark moments of the show actually hurt it in the long-run. Without Rita the plausibility of him being able to be the murderous Dexter started to fly out the window, and the contrivances started.
I can really tell you the moment Dexter really jumped the shark, though. Lumen. Once season 5 started to get into full swing and we learned that this wasn't Dexter who was on the hunt, but someone else he was simply assisting, it started the real and visible descent into becoming a bad TV show. Not only was there no Doakes, no Rita to support Dexter and make the show interesting and grounded when he wasn't on the hunt, but even now the big baddy wasn't so big at all. No one gave a shit about Jordan Chase because no one gave a shit about Lumen. Chase wasn't a bad guy they established like Trinity or Prado. You didn't care about him because likely if Dexter never meets Lumen he and Chase never mix it up. No one cares about him because Dexter didn't care about him. No one cares about Lumen because the writers were trying to shove this odd love story down the audiences throats when everyone is still shell-shocked from the finale of season 4. It was also the beginning of the writing seeming to flounder and not knowing where to go with the show, like the creepy babysitter lady stuff with the Jesus statue, and so on. Once season 5 ended with everyone all hunky-dory, with them trying to convince you everything was OK and back to normal, it in fact was everything but.
The writers have written themselves into a hole over and over again, and are left with nowhere to go with the character. Deb knowing is inconsequential really. It's something that happened in the first book. It was never something Jeff Lindsay meant to have hanging over his series, rather a twist in the first book of the series and an facilitator for Dexter's actions later on. In fact if they had replaced killing Doakes with Deb finding out not only would it have facilitated keeping Doakes around, if and when they did kill Rita it would have facilitated Dexter plausibly getting away with being an absentee father as well, instead of him magically having a babysitter that is more of a full-time mother than a college kid watching a baby for a few hours. Instead we're left with a cliffhanger everyone saw coming a mile away, one in which no one really gives a shit about the outcome.