This is a great thread and touches on one of the more prominent issues that I think Sims 4 struggles with.
The emotion system is definitely a major catalyst for a lot of repetitious and predictable behaviors. Not only do all Sims emotionally react the same way to the same "stimuli", but they also cycle through a few of the more prominent emotions more frequently due to excessive buffs, so even the general mood of each Sims seems to lack diversity. This can also affect the selection menu of actions and conversations, so we end up with a bit of stagnation in our options for Sim interactions. The result is watching them repeat the same animations, the same facial expressions, the same reactions (or non-reactions,) etc. There are very few surprises
One of my personal pet peeves is the water drinking habit. I just can't imagine why this feature was even considered to begin with. Throwing in a hidden thirst meter that forces Sims to constantly wander across the lot to retrieve water on a regular basis, just seems very redundant when paired with a hunger need. It does nothing to enhance gameplay and has, if anything, made me realize that too much "realistic" behavior can actually cripple a game more than enhance it. Especially when we already have Sims flocking to the juice bars. On top of that, the "feature" gets bugged and all of the Vampires are suddenly flocking to the sink to retrieve water. I truly wish they would remove it completely.
The last one I want to mention is the idle and pre-programmed responses. Greetings and farewells are all the same, no matter what traits, relationships, or behaviors you've given your Sim. I've had Sims purposely pick fights to try to build bad relationships with siblings and strangers alike, but as soon as I stop directing their actions, they are hugging it out, smiling, pleasantly conversing, and being...well...boring. I can give my Sims traits like Evil, Mean and Hot-Headed but it doesn't actually seem to dictate their behavior. I still have to push them to give them believable personalities and if they don't create enough buffs to counter all of the effortless positive boosts, they act like every other Sim. When left to their own devices, Sims 4 Sims have no individuality. And that's one of the most disappointing aspects of Sims 4.
