First off, the name 'The Hammer of God' in reference to an asteroid that could destroy mankind is evocative and a perfect tone-setter for Clarke's writing.
Once again Clarke manages to create genuine characters among the hi-tech of the future which manages to ground the book in a way that many modern sci-fi books lack.
The tone of the book is very much one of impending doom and Clarke switches between the doom of Earth and the immanent self-sacrifice of the members of Spaceguard. I'll admit I was surprised by what happens to the crew of Goliath at the end.
The story is driven by two things, the past and present of the Goliath's captain and the desperate efforts to avert the disaster. The captain, Robert Singh, leads an endearingly tragic and lonely life that is mirrored in the characters Heywood Flyod and Frank Poole from Clarke's Space Odyssey series.
Finally, my favourite part of the book are the various 'Encounters' which chronicle Earth's past meteor impacts and warn of future ones.