The threat posed by Peter Weller was utterly undermined by the conclusion of the season. Dexter stabs him; Quinn is wrongfully accused but acquitted by Dexter. (Not only is the acquittal totally implausible, but the way Quinn shifts to being totally buddy-buddy with Dexter doesn't make a lick of sense).
Likewise, any interest in the Lumen character was undermined by the way her Dark Passenger suddenly vanished and she just left in the finale.
Deb's story was undermined because the writers didn't have the balls to go through with her finding out about Dexter. A giant plastic sheet in between them is awfully convenient, but Deb walking away after her speech is several orders beyond. Also, I recall a female uniformed cop from that season who did something bad to be in Homicide, but once she got her spot in the department her character was completely dropped. What a waste of time!
Mostly, it was a waste because it completely squandered the death of Rita. Dexter's nanny was nothing but a plot device. The nanny from season six is basically that, too, although she has had more connection to the other characters. Quinn was never a threat. Season six has done more with the death of Rita than season five, I think (outside of the season five opener, which was a terrific episode, but one that didn't lead to anything).
I think season six is better because Mos Def's Brother Sam was the most interesting character on the series since season four (although he came perilously close to being a magical negro on his death bed, and he was killed far too early). The Doomsday Killings have at least been unusual (I forgive the implausibility of how elaborate they are because they've been so unique), and unlike Peter Weller, Colin Hanks' villain has been mediocre from the beginning, so nothing will be undermined by a weak conclusion.
Yeah I agree the payoffs
to those stories were pretty lame, but at least it still felt at several points in the season that Dexter was at some kind of real risk
. Whether it was being caught actually killing someone, having to cover the tracks of a bumbling Lumen, having to deal with Quinn's suspicions, or having to evade Liddy's prying eyes (eyes which not only saw them dumping body parts, but practicing to kill someone!). And yes, even the possibility that the nanny might not be all she said she was.
For whatever reason, it just hasn't felt like the stakes have been very high at ALL for Dexter this season. Other than the vague possibility of Brother Sam learning the truth, he hasn't been under much threat at all. And that to me just makes for a very dull season.