Why would a pilot's armrest be a no-go item? From the end of issue #216 (June 1997) of Callback:


  Another First Officer also learned too late the importance of consulting the MEL before take-off.
  
  
    Inbound equipment arrived late. About 5 minutes before pushback time, I discovered that my right armrest was stuck in the UP position. I attempted to free it, but no luck. I contacted maintenance and asked them to bring something to lubricate the mechanism. A mechanic did so, but to no avail. We figured we could just go with it in the UP position. It did not occur to us to look it up in the MEL, as it did not seem to be a safety-of-flight item.
    
    Upon our arrival, we were told that the armrest is a no-go item. The continuing flight was cancelled due to this fact, since no spare seat or replacement part was immediately available. The Captain and I were both amazed that something as simple and innocuous as an armrest would be considered a no-go item. In the future, I will check the MEL for any item, no matter how seemingly insignificant.
  


Why would a pilot's armrest stuck in the up position be a danger serious enough to require grounding the aircraft?  One would think, like the aforequoted pilot did, that it would be, at worst, a minor inconvenience, and certainly not a safety-of-flight issue...
Perhaps it was just an oversight on the part of the creators of the minimum equipment list.

As I understand it, minimum equipment lists generally provide a list of those items which are permitted be inoperable. So if any piece of equipment is inoperable, and it's not on the list, that automatically means you're not allowed to fly.

(Going by the name, you'd probably expect a minimum equipment list to be, you know, a list of the minimum required equipment. But for one reason or another, it's not.)

So, in all likelihood, nobody thought to add armrests to the minimum equipment list. As a result, flying with an inoperable armrest was automatically prohibited.

However, things have since been adjusted: the FAA has added the armrest to the master minimum equipment list for the Boeing 757. So, it is now legal to fly your 757 with a stuck armrest, as long as it is stuck in the "up" position.