This is the reason why all my sims behave the same. I'm not sure why the developers went from 5 in TS3 to 3 in TS4. In real life someone may have around 20. Now 20 would be a bit much for this game, but I think we could use 5-10, depending on the function of the trait. I believe traits are too focused on triggering emotions. It could use more unlocked interactions and autonomous behaviors.
A hobby trait can pretty much be unlimited when it comes to how many you select, as it makes a sim better or more inclined to participate in an activity.
An emotional trait is also pretty much unlimited. When a situation occurs, it triggers an emotion on a sim. There may be a few conflicts here and there such as 'cheerful' and 'gloomy'.
Lifestyle and social traits would be a bit more limited in the number you select. If you pick a lifestyle or social trait, it is more likely there will be others that will conflict. That is ok, as this is what makes each sim unique. You would not have someone evil and good at the same time.
I think sims 3 had a pretty good system. My suggestion would be to introduce new traits and increase the number of trait slots per sim to a minimum of 5. I discussed this whole thing
Here
