Well, three complaints:
[spoiler]1. The plot relies on Luther being too stupid to delete a text message with Mary's location on it.
2. The plot relies on the hoary old "I'll give you the tape" cliche, despite the villain being fully-established as computer-savvy and someone who should understand the concept that files can be copied.
3. The resolution to the Internal Affairs plot is far too convenient and facile.[/spoiler]
Aside from that, I enjoyed the hell out of it. Predictable, but that was kind of the point; this show is about coincidences, repetition, and the lingering questions of what would have happened if things had gone just a little bit differently.
So really, it's inevitable (even though I'm spoilertagging it) that season 3 ended with [spoiler]a dead partner, the woman Luther loves being used as a pawn against him, a rooftop showdown involving a vote to decide who lives and dies, Luther's desperate desire to see the villain live to pay for his crimes, and the antagonist investigating Luther starting to believe that maybe he's not so bad after all. (I'm glad Erin lived, curious as to whether their relationship will be positive in the future, but kinda hoping it remains cold if slightly less adversarial. I expect she'll continue to be involved somehow, though, because Jesus Christ they've sure burned through a good big chunk of the principal cast at this point.)[/spoiler] Course, it bears adding that Luther is less morally ambiguous than ever, played things pretty close to by-the-book this season, and seems to have finally gotten that whole anger management thing under control.
Curious what curves the show has left to throw at us if there's another season; hoping they don't amount to ramping up the shock and misery because this season got pretty grim even considering what's come before.