I'm watching this through a second time about a decade after my first go through. I'm surprised how well it holds up.

The other reviews have covered everything but one thing that popped into my mind this time is that another name for the Devil is the "angel of Light".. thought that was an awesome detail. I'm not sure if it was deliberate but given how well thought through this series is I wouldn't be surprised. The series even opens with a version of the piece "God have mercy" (as seen in the trivia section) so there are plenty of layers to explore.

Of course the last 12 or so episodes leave a lot to be desired, but still worth watching. There's a reason it's a classic.