And while the Merlin series has been conservative in most aspects, Merlin 1D has a few unique characteristics, like the pintle injector (previously used in the Apollo LM), phased-shutoff (or whatever the spelling is, from Tom Mueller's interview from half a year ago), use of subchilled propellants and a very tightly manufacture process that makes it very cheap to produce.

On top of that it's now the engine with highest thrust-to-weight ratio in history that has flown, despite being open cycle (gas generator). And not a single M1D failed so far.