I've been having a discussion by email with one of my players about disadvantages. I thought elements of that were worth sharing and preserving on the blog.
In DF Felltower, some disadvantages warrant special treatment because they're especially disadvantageous, or because they call up circumstances that just don't occur often.
Here is how I priced one:
Vow: Never refuse a one-on-one challenge to combat
Much like the -10 point version, you've sworn to never turn down a challenge to combat - but only a one-on-one fight. In DF Felltower, this is a Quirk.
It's exceedingly rare for a PC to be challenged to a specifically one-on-one combat. It's happened twice that I can recall.
The circumstances are very unusual. You'd need a foe that wants to fight a specific PC. The that foe would need to make some kind of effectively communicated challenge that the PC could understand. Then, the NPC would need to face the PC one-on-one. It's not clear who determines what "counts." Presumably the Vower. The vower can't say no, so it's tough if the NPC is, say, the Lord of Spite. It's hard to rule on if the foe is a dragon who roars out a challenge at the PCs in general or the PC in specific - was that a challenge to a duel, exactly, or just a general roar?
Also this disadvantage also says nothing about help. The PC can be buffed and enchanced by friends. Can the NPC? Does it still "count" as one-on-one if the enemy has magic put on him? What if he puts it on himself, still fair? What if the PC accepts the challenge, but then the PC's friends jump in? If the PC refuses help, that's nice, but it's not really that restrictive unless he's required to give up existing advantage as well as turn down potential help.
So the circumstances come up rarely. The vow is minimally restrictive even when they do come up. The vow doesn't really hinder the PC in any way except when it turns out a really nasty foe makes a very clear challenge to fight.
Hence, this is a quirk. It'll come up - even quirks can have very steep game effects - but rarely. When it does, it won't really restrict the PC's actions overly.
In a different game - perhaps a Musketeers game inspired by Alexander Dumas - this could be -5. In a megadungeon with group-on-group fights with magic around, it's a quirk.