Every single episode of that season was unpredictable yet well developed. We understand the basis of the main villain in Trinity, but we are unaware of who exaclty his next target will be, and - later on in the season - how close they will be to Dexter. At the same time, it does a great job of developing tension and unpredictability by having Dexter get so close to Trinity. The final moments that mark a huge death in the series - coupled with how the police station was successfully diverted from the real story - are a great example of something that was not trite, easily predictable, or too basic.
But then there's season 5. The premise is there from very early on: there's a dude with an evil group of people who do shitty things and Julia Stiles wants them dead so Dexter will team up with her and kill them and it will be covered up and then she'll probably leave or die because she's a guest character. This is also the season where the entirety of the writing gets really bad. The ending of that season where Dexter gets all angsty about her leaving was out of character and plain not fun to watch.
Season 6 gets worse. EJO is clearly an extension of Travis Marshall's mind - the way it's set up in the very first episode brings that thought to mind and with each consecutive one it becomes more and more obvious. The entire trope of religion and forgiveness is forced. We know Dexter will kill Travis and with Debra's promotion we also can finally begin to suspect she's going to find out because she is more involved. Add in terribly written plot lines for the side characters, over-the-top killings, flat out unbelievable escapes by multiple characters, and you've got the worst season in the series right there.
Season 7 has already made it abundantly clear that the new dude is going to either take the fall, get killed, or both. The fact that we know this isn't the last season does, to be fair, have a decently sized role in this. But he's overstepping boundaries from the beginning and he has all the makings of a perfect scapegoat. And then you've got this Ukranian plotline that's over the top and almost certainly irrelevant to the overall development of Dexter and the series-arching storyline.
Which brings me to another point: the last two seasons have had absolutely no impact on Dexter's character. The sixth season tried to set it up so that it would, but everything that Brother Sam worked towards went to shit after he died and is now completely inconsequential. The Season 5 "can I love?" BS right after Rita's death was forced and also completely inconsequential now. However, now that Deb knows, this opens up a CHANCE for them to do make a reasonable development or change in his character. Do I trust the writers to at this point? No, not really.