The most important stuff (Score 1)

An applicant's personality and general method of working are usually more important than knowledge with any application, system or standard. Some knowledge is always of importance, and for some positions it might even be crucial. But knowledge can easily be overrated, and are in close to all cases of secondary importance (above a certain level; if you can't tell a DIAS-scanner from a toaster, you will not make a good sysadmin no matter your personal qualities).

Say one candidate has most of the general skills needed, obviously takes pride in his work, has ambitions and are very enthusiastic about the position, whereas the other is an expert on all relevant topics, has longer relevant working experience, are a bit laid back and has no long-term professional goals.

I'd hire the ambitious applicant any day.

Working is thinking followed by action. If you have extensive knowledge on some topic, you need to do less thinking to do tasks related to that topic. You still have to think, to some degree. The ambitious person, eager to solve the problem, do a good job and get a raise, will enthusiastically push his mental throttle all the way. The expert run dry will give up or postpone the issues as long as possible.

Knowledge is simply the result of having thought about something. A person that knows a lot has a good track record with regards to thinking. But the fact that you were taking good care of your mind five years ago doesn't mean you do today. That's why personal qualities, and especially ambition, is an important element when judging how good a job a candidate will do.

"I swear by my Life and my love of it that I will never live for the sake of another man, nor ask another man to live for the sake of mine." John Galt in Ayn Rands "Atlas Shrugged".