My company is growing at such a rapid rate that I'm tempted to start promoting people based on seniority. I know that is not a perfect solution but it sure would streamline the promotion process and allow us to focus on more important things.
-- Name withheld by request
Promoting based solely on seniority is not just an imperfect solution--it's a terrible solution.
Plus it's also a cop-out: Everyone I've ever talked to who promoted based on seniority eventually admitted they did so not because it was easier but because it allowed them to avoid having to tell employees why they weren't selected for the job. So if that's the real reason you're thinking about it... shame on you. Your employees--especially your great employees--deserve better.
I have a bunch of problems with seniority-based promotions:
I would also argue that there are no more "important things" than how you treat your employees. If you think it's too much trouble to know your employees, to know their skills and talents, to know their strengths and weaknesses and put them in spots where they can showcase their strengths and improve their weaknesses, then you need to get someone else to run your business.
And if all this doesn't convince you, here's one more reason. As a start-up your company competes with plenty of businesses with greater "length of service" in your market. You'd be royally pissed if a potential customer said, "We're going with ACME because they've been in business longer." In your mind, customers shouldn't care about the fact you're new; they should focus on and choose you for the simple reason that you are better.
Shouldn't you treat your employees the same way?