The thing that I’ve worked harder on than anything else in my life. The passion that I have spent the last seven years toiling away at including blood, sweat and tears: dressage. Just doesn’t apply to my office drone day job. Although, if I could bring my favorite horse tool into work, my whip, things would run a lot more smoothly around here.
I’ve organized other conventions with similar attendence that are actually professional - so I have other “able to organize an event” bullets.
Why? That’s excellent experience in dealing with managers successfully. 
Another person who has (and still is) working on science fiction conventions at various levels for the last twenty-plus years. When I have applied for promotions at my former job and had to submit a resume I had no qualms about including the work I had done for conventions, particularly that for several Chicons, for the same reason cited by RealityChuck: it demonstrated my ability not only to organize an event, but also demonstrated a familiarity with procedures and personnel matters which were not part of my “regular” job.
I have two resumes, one for fiction writing and one for my profession. Pretty much the only thing they have in common is the name and address at the top. Not sure if that’s what you’re looking for, though, so let’s see . . . .
I worked as a nanny for a year for a muckey-muck at a Forture 100 company; that’s not on either resume.
Eliphalet, Dangerosa, RealityChuck-would you mind checking out this thread?
Some stuff to leave off:
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Took the regimental gold watch for “Soldier of the Year” in 1987. I’m stinkin’ proud of it, but it’s irrelevant to Network Engineering.
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Was part of Mismanagement for a handful of national and international hashing events - including the Aarhus Festival Hash, the Danish Nash Hash, the German Nash Hash (in Denmark, a bit of a rarity) and the Inter-Scandi Hash. Hashers despise authority, yet expect high standards as regards organization and logistics.
Bookkeeper for an escort service, which resulted in me helping the owner to find out 3 of the girls were skimming pretty consistently. I leave the whole thing off even though it was an 18 month gig.
Oh, I also leave off the accomplishment from when I installed a security system at a supermarket. Turned out the night manager liked to watch the count room because 2 cashiers would use it for fornication.
I leave my telemarketing job off my resume, even though I was working there a year and was pretty good at it, because I hated the work and would only be able to speak negatively of it at a job interview. The place doesn’t exist anymore, either, so references from there would be useless.
I also don’t mention that I’m a demon in the sack. That would get me the wrong sort of job offers. 