With the addition of all the user made traits, the box we select them from get's full if you download all that are available. you can not scroll the box down to select ones that go off screen if you have a lot of them installed. Now were not even to the first expansion yet and the box is full to the point of not being able to select traits, I imagine that EA is gonna add more traits themselves pushing more installed traits off screen. So the addition of a mod to either widen the box of traits sideways to make more columns, a scroll bar to reveal off screen traits, or just make the icons smaller to fit more on screen is sorely needed.

Bumping this up because I too see this as a problem. I was looking at custom traits earlier today but held off downloading based on reading this before knowing some will be inaccessable

I noticed this was a problem too. I found a little work around. The category that the trait falls under doesn't seem to influence anything significant. A trait's a trait. So you could take a trait and just change the classification from, say, Lifestyle to Social. It's not a real fix to the problem at hand, but it will help people like us out until a real solution becomes available.

but there's sooooooooooooooo many good custom traits. xD I wonder if we can delete EA's traits we'll never use, like "Bro" can someone make one of these custom traits into "bro" override? xD I hate people like that irl, I hate them in my game.

There's a temporary fix posted here. It's not ideal but it works for the meantime.
http://www.modthesims.info/showthread.php?t=549529

How do you re-classify the traits please?

Since I was the one who first tried this to resolve the problem, I can say in GTW, it doesn't work. It ruins taking photography due to a funny uiscale :/ it's a shame.