I suppose the third is people who got a degree, but never really understood how things worked and the just didn't love IT..
You need passion for this job, I cant think of going home and not keep working on some thing that find that day and was unable to fix/or figure out.
I think that the constant change and the need keep your skills up to date are the things I like about the job. (I always try all new microsoft release to find how to fix the new features or was inly a SP whit new name and price tag.)
and working using Microsoft software make you an adrenaline junky, ok the days of nt4 and Exchange 5.5 are over, but to maintain a no downtime for years (only scheduled) on complex networks (Clusters/Exchange/sql/etc) ore small ones whit no budget is a challenge that require much commitment on your part.
many times you have to protect the enterprise from the owners decisions. and not to mention make them understand why change hardware before is broke (life span of X hardware 5 years, I request change after 2.5 even if is working fine).
And the better you do your job, the system work whit no downtime and go and ask for a raise....If you don't do your work and they have problems yes no problem...
Today in argentina is hard get people with the passion of my generation (I have 39), and is normal chat between old coworkers, we don't see the same commitment, they stay in they comfort zone, they even don't want free training.
I have seen IT personal whit no skill, but I notice the ones that one to learn, and I take care they have all they need, even if require my own time, but that is very rare today.