The character defining aspect is how it is most used in my game. Our Fellowship has a Hobbit lass who smokes a pipe, treasure-hunter of sorts but she's after spoken treasure as opposed to physical objects. Tales and stories are her obsession, collecting, recording, and preserving. The 'smoking' trait she uses when dealing with recalcitrant LM Characters. Once she failed an Encounter, speaking for the whole group and blamed it on her smoking. Their elven host was not so enamoured of her smoke rings as the Beornings they encountered a fortnight ago. (Actually, the Beornings were more amused by the little lady smoking than anything else.) She had no Advancement points in the Custom skill group. She invoked 'smoking' in her roleplaying the failed roll, and I awarded her an Advancement point. The failure was significant and the use of 'smoking' as a reason was priceless!SirKicley wrote: It's important to remember that having a trait isn't required to be able to know how to do it. A trait means that it's a character-defining aspect of you. It is so much a part of you that it can lend itself to a number of areas that most other smokers would not.
Another use that arose within my Fellowship is to judge the quality of pipe-weed. Two character's have the trait 'smoking' and another acquired a pipe for smoking, but doesn't have the trait. This in itself plays out differently, the two with the trait are...well, clearly different from the Barding who just owns a pipe and merely smokes. The Barding doesn't know good leaf from bad much to his chagrin at times.
As Robert points put, these are defining characteristics that transcend mere benefits to be leveraged. They can explain both beneficial qualities in one episode and, in the next, be the reason a player-hero blunders.
One of my players wonderfully led (blundered) into a 'Change Distinctive Feature' during a Fellowship phase. He often invoked a trait as the reason for his failing Custom skill rolls. A sort of sub-narrative began amidst the players that his trait was more a hassle than a benefit. So he had a reason (narrative as opposed to mechanics) to change the trait. This wasn't so much planned as evolved (with a little mentoring) and, in the process, he earned a degree of respect for himself amongst the Elves of Mirkwood.
Just some additional thoughts to add to the mix...
Regards,
Scott