Hiring speed, hire quality, compensation (including things like work/life balance, healthcare, flexibility). I think you can try for two.

Once you get large enough your ability to really selectively recruit gets a lot harder when natural attrition means you need to replace X amount of people just to continue operating as before.

Of course earlier on, the fewer the people the larger the impact each one will have so just like the article says about transitioning to enterprise, the focus and requirements of the HR team change as well.