We're finally going to replace our sysadmin that quite a while ago, but that means we're back into interviewing, which is a lot of work. Trying to figure out if someone has the skills that you need and if they have the personality that will fit in is very difficult.
One aspect that I find particularly challenging is determining motivation and speed. Will the employee work quickly, or will they take forever to get things done. Yeah, they're technically competent, but will they use any of the knowledge that they have to get stuff done?
I tend to be a difficult interviewer, grilling them for about an hour to try and evaluate their personalities and such. I wish I had more time for the interviews, but that's rarely the case. Oh well, it is what it is.