Donkerbos (translated roughly as "dark forest") is different from your average run-of-the-mill detective show. Yes, it is slow-paced, but the story and various sub-plots build slowly and the characters are developed further with each episode. The story itself is quite dark and the lighting and cinematography serve to reinforce that.
The actors all do a good job, in particular, Erica Wessels (Fanie) and Nicole Holm (Marietjie). Some of the scenes, in particular, the final episode, must have been quite tough to film. Yet, all the cast managed to play their parts well.
In relation to the viewer who complained about the language. The show is based in South Africa, where both Afrikaans and English are spoken as well as several other languages. So it is perfectly natural that the spoken language will change depending on setting or who is being spoken to. As for "badly-accented English", I only heard South African accents. Perhaps the reviewer has never heard a South African accent before?
If you're looking for a detective/mystery series with a decent plot and some substance, then Donkerbos is a very good choice. It is a refreshing change from the below-average detective shows being pumped out by both the US and UK. Hopefully there will be further seasons in the future.