Character development threads are for just that... character development. What goes into each varies with the writer and the character. For some it's a laundry list of kills for the day, for others it's a peek into the inner most thoughs of the character, for still others a mix of both.
A Character Development thread will be a necessity for the movement into several of the prestige classes, as it shows the character's motives for going in that direction (or it should)
A good Character Development thread, kept updated regularly will also help those thinking of applying for Epic levels, rather than trying to cram one together at the last minute to show your proposed epic character's epicness.
For those with a creative flair it's a great and fun way to flesh out a character
Keep in mind though ... do not abuse the fact the this is journalistic. Boasting about exploits not done in game or things that if done in game would have resulted in serious consequences should not be done. GM's simply do not have the time to read everyones journals. As a GM I do read those of characters that catch my eye, or randomly go thru them on a slow day.