I'll add my own opinion on the title, though:
A personality trait, and a lot of other things in this thread, can be a pretty big weakness, but a lot of people use it in their character biography as a throwaway to get past the two weakness requirement. I don't blame them, though, so it's not something I think about doing a witch hunt about.
I believe that the Strengths and Weaknesses section should be used to essentially give a short summary of a character. At the least, it presents four core traits of the character that are nearly always shown. Every significant character has some weakness, and most likely some strength - even those in Star Wars movies had some personality traits as weaknesses. Difference between the movies and here is that the characters had displays of their weakness at nearly every chance given. I honestly think most people forget about those throwaway weaknesses shortly after character creation on this site - so something such as pride never gets shown in actual role-playing to be a weakness.
In my opinion, a good way to present a personality trait (and any other) in a biography is to present an example in which it causes a significant issue. Daella's biography showcases this - I give the reader something that demonstrates the weakness's practical implications.
To make it perfect too, the personality trait has to be a foundation in the character's being - just like any other person's personality traits. I'm fairly sure that everyone that has interacted with Daella via role-play understands her core personality traits and what could be assumed to be a key weakness in them. It has a distinct, unmistakable effect on every social interaction that Daella has and has even prompted characters to respond negatively to it - thus solidifying it as a weakness worth being listed and kept.
The same could be said for strengths. They're very core for Daella and are displayed in nearly every thread as often as I can just as weaknesses. It gives the character flavor and depth - even at the most basic levels - and helps begin the process of creating nuances in the character.