Oh, so many mistakes were made. The glacial pace of releases for supernatural content, the lack of any sort of Create-A-World or one-click world clearing, the difficulty nerfs, the insane length of the dog walking action (the "short walk" takes, like, 3 sim hours!), DLC for DLC, several base game features not being present at launch or being withheld for packs (most infamously, hot tubs), pandering to players who are so bad at videogames that they don't even know how to save scum (I mean, that's one of the first things you learn to do as a gamer: save often, make safety saves, and exit and reload if you don't get your way...), the snowball/water balloon fight animations...
However, I think the biggest mistake was the decision to give us the barest minimum of gameplay options. It's the reason for the difficulty nerfs, it's the reason why supernatural gameplay has been so heavily neglected, it's the reason why there is a constant tug-o-war between players who want to be rotational puppet masters who orchestrate their sims' whole lives and players who like their sims and worlds to progress without them when they swap households. A robust menu of options is extremely important in a game like this, where players are allowed and expect to play God, to give them the options to set up their game however they please. Mods are alright, but they tend to break after updates and can be a chore to maintain. Mods are not the solution to a lack of built-in options, they are a Flintstones bandaid over a ten-inch-long, two-inch-deep gash in your leg when what you really need is stitches. Those stitches, in this case, would be patching in advanced gameplay settings.