in reply to Certifications are dumb.
And, most importantly, I need to know how well you communicate and how driven you are.
In my experience the last thing you listed - how driven a person is - is practically the only thing that matters. Sure, it takes a certain amount of native intelligence and attention to detail to do a good job, but without some kind of inner drive that won't do it. I'd take a self-motivated, eager beginner over a more experienced slacker any day.
The problem is, how do you tell? Most people manage to put on a good, chipper face for interviews and many can sustain it for months on the job. True slacker-hood rarely shows its face early.
I think you need to be ready to cut dead weight quickly and mercilessly. Your good employees deserve it. Don't be drawn into endless "come to Jesus" meetings - they just don't work! If your company is too big for you to personally know who the slackers are that means you have to empower your technical leaders with the power to terminate, and encourage them to use it.