The absolute biggest problems to me are not the technical limitations self-imposed by the developers when they picked a sub-optimal engine (again, I absolutely believe them when they say many things we want are very very hard to fix or do, but it is their FAULT because they chose an engine not suitable to create Sims in). The biggest problems are not the bugs, or the musical chairs or whatever.
The biggest problem is the absolute paper thin personality system. The perks are basically pointless, the emotions are as fickle as a leaf in the wind and there are nothing but the perks to actually define a personality.
"This sim is unflirty and a loner. That means it takes a whole day to go from 0 relationship points to marriage instead of half a day like other Sims". Seriously, there are NO meaningful traits, period. Exept maybe Vegetarian because it can actually force your Sim to throw up. So that's actually the trait with the most jarring consequences.
Combined with the absolute lack of challenge (Seriously, how long does it take to get rich in Sims 4? An IRL afternoon?) it makes the game boring. I remember reading that according to collected data most Simmers don't want a challenge, they just want to tell their story. At least that was the explanation back in 2014 why virtually all challenge is removed and also why the Sims don't have a personality; you are expected to make up their personality in your head, the Sim is at all points a blank canvas without any real purpose other than to wait for your orders.
