Out of the Adventure Paths context, I’m not a great fan of Character Traits. They were created originally for the awesome Crimson Throne Adventure Path, giving little mechanic benefits for backgrounds linked to one of the campaign’s NPCs. The idea quickly took root, showing up in future Adventure Path Player’s Guide.
These Traits are also called “half-feats”, which gives a good idea of their mechanical potency.
The Advanced Player’s Guide officially introduced Character Traits as a generic mechanic. And that’s my problem: without any connection to a campaign, I believe that Traits are practically useless, just more “rules inflation” on character sheets… but then I remembered Wolfgang Baur’s excellent idea about giving Aristocrat levels as a special reward (in the first Kobold Quarterly).
Character Traits could be an interesting way of giving lesser mechanical due to an adventure’s consequences (not always good consequences, please note that). They’re very specific and traditionally linked to campaign’s places, legends and persons. In other words: perfect. They could easily represent unique training, mystical initiation or even a bitter defeat.
Just another random thought provoked by long tedious hours of study.