One more thing: Challenging/important tasks.

Some of this gets filed under "Growth Prospects", and some of it gets filed under "Autonomy". If your employees have challenging and/or meaningful things to work on, and an environment where they feel they'll be successful, that makes it quite a bit easier to stick around.

Not to say that grunt work is bad, but your employees who need a larger motivation than "It's my job" or "It's how I earn money" need to see how the work they do contributes to something larger than the stack of paperwork on their desk.