And Then There Were None is downright *creepy!* Murder on the Orient Express was somehow quaint and charming - I think Poirot was a comforting figure to have around. But this one is just... madness on an island.
Perhaps I only notice this having already seen the Unicorn and the Wasp, but I'm very glad they did a sort of Dr Who/Agatha Christie crossover episode. The formats snug together very well. For one thing, there's always a trustworthy Doctor to compliment your Detective. They form the dynamic duo at the heart of the action. Then you must have someone slightly less brilliant than they to serve as their foil for the audience's attention, and you have a ready-written role for Donna. Then all you need is a cast of characters and a relatively isolated setting to act as dressing for the crime. Easily done by writers so accustomed to crafting one-shot characters and locales as the team behind Doctor Who.
As for weaving aliens and time travel into the proper, civilized world of 1920s England, with Surprising Turns of Events already Dame Agatha's stock and trade, it was no great feat to find things for the Doctor to do. And having Agatha Christie herself show up and star as the detective immediately sets this in the "real world" of Doctor Who. (I know, to say that Dr Who is closer to the "real world" than Agatha Christie's perfectly realist contemporary novels is bordering on the absurd, but Dr Who likes to *pretend* that it travels in our actual history, present, and future. It adds interest by utilizing historical figures - Shakespeare, Agatha Christie, Charles Dickens, Madame de Pompadour - and then allowing itself to also include aliens and say "Look how weird the world might be! There are just as many aliens in the past as in the future!" That allows it a much broader range of episode styles - the historical episodes are definitely distinct from those in the far-future.)
Dr Who frequently has a mystery flavor to it - suspicious circumstances, guilty parties. Whether consciously or not, and certainly not strictly, it occasionally finds its roots in the archetypes of the mystery genre, and Christie's novels are certainly that. The Doctor plays detective quite frequently, right down to the plucky sidekick. Really, as I said before, a very good fit.
Disclaimer: This is probably the second true "mystery" novel I've ever read, so all this is pretty much talking right off the top of my head. Call it a record of my initial impressions.
Anyway, I'm quite enjoying the contrast between these two novels. I worried that I might tire of Christie rather quickly if it were all generally the same story, but I don't see that happening yet.