It's mainly the lack of traits yes. But they can fix it, sadly they never did much with traits since launch. Few traits have been added, and mostly it was just some simple moodlet thing like cat or dog lover.
What they need to do is work on adding like 30 new traits and increasing the number of traits a sim can have. At least to 4 and possibly back to 5.
Of those 30 traits, most could be returning from sims 3. I want new negative traits too, to make it harder on sims. They are too darn nice to each other! While that's a positive message and all, we need some drama! Some new trait suggestions:
-Repulsive.
This sim is seen as ugly by others, making it harder to find romance. Sims with the good trait could ignore this affect however, improving that trait at the same time. Friendship should work well enough by default, but snobs would be reluctant. Mean sims are able to make fun of their appearance.
The upside is that it could scare ghosts away, they get extra appreciation during halloween and such. Getting into some jobs could be harder for them too, anything that requires them to be the face of a company.
It's not tied to looks however, you could make a supermodel with this trait. So what your sims find repulsive is up to you. Could just be people wearing ugly christmas sweaters or a specific haircut.
-Disloyal.
These sims can't be trusted in jobs, relationships or friendships. They'll toss anyone under the bus if they can gain from it, they get positive moodlets from that. It's like a worse version of commitment issues. They aren't sad if caught cheating or when seeing their partner cheat however, to them it's the natural order of things.
-Masculine.
These sims act more masculine in various interactions such as flirting. A lot of this is in body language. They favor more masculine activities and won't do more feminine activities on their own. They aren't against other sims doing that, it's just not something they'd do themselves.
-Feminine.
The counterpart to the masculine trait, these sims move more feminine in various interactions and favor more feminine activities and won't do more masculine activities on their own. Both of these traits can be given to any sim, but they obviously can't have both traits.
-Likes a drink.
This sim needs to drink. They want a bar, they want to drink lots of drinks. They get a negative moodlet if they haven't drunk anything in a while. Which get's worse if they keep not drinking for longer. Could be like 3-4 stages of this but if they get past it and thus overcome their habit, you get to select a new trait for them to replace this one. But it will be rough especially in the final stage with a huge negative moodlet.
Seeing sims overcome this might help people dealing with this problem in real life.
-Fool.
These sims aren't too bright, but easily amused. They learn slower and can do some crazy things like prepare unique meals of weird combinations that only fellow fools, slobs and gluttons can eat without getting sick or feeling uncomfortable from a disgusting meal.
They can also do various 'check this out!' interactions with other sims to try and gain their attention.
They are quick to see other sims as their friends which is a bonus however, but also have some low brow humor like 'pull my finger.' that snobs will detest. They can also interact with some objects in ways no other sim can, doing something crazy with it.
-Spartan.
These sims prefer a simple life style and do not like luxury. They get no negative moodlets from cheap beds, showers, fridges and such, but instead get negative moodlets from needless luxury. They do enjoy things like reading books, talking with other sims and building skills with less expensive objects. They actually build skills faster that way.
Some skills are harder for them to gain such as programming, video gaming or such because they'd have a negative moodlet while using such needless things.
-Loves water.
These sims enjoy water like no other, and get unique interactions in swimming pools, bathtubs, showers, hot tubs and at sea.
If there's a pool, they are in it.