The shorter version of The Descent goes something like this:
Indeed, there are lots of places where the movie shines: it's well-acted, solidly directed, and written intelligently with a set of strong and interesting female characters--a rarity in horror. But its real strength is that it maintains a constant level of tension and dread for practically the entire film, yet doesn't overstay its welcome. It does this by layering and gradually introducing new stresses, starting with a vague but definite unease between the main characters. Once in the cave, that unease is magnified by the claustrophobic confines and some clever tricks of the light--at times the characters are lit as normal for a film, however unrealistic, but in other cases the frame is almost entirely black, with only a few outlines and bobbing headlamps visible. It's only when the audience gets adjusted to the dangers of spelunking that The Descent introduces deformed cave monsters to the mix. With a running time of less than 100 minutes, there's just enough time to develop the scares, but not enough for "villain fatigue."
All in all, The Descent is one of the best horror movies I've seen in a very long time. It's not particularly original, but it displays a mastery of the genre and pacing that similar niche horror films (including the ridiculous Hills Have Eyes remakes) would do well to study.