While the graphical improvements and general overhaul of the core mechanics were a welcome, the illusion of the massively over-exaggerated "Free Will" engine and metric ass-ton of problems made the game almost 0% enjoyable for me. The problem with the Free Will engine? It just makes the Sims randomly do things. Their actions aren't determined by mood, various meters (bathroom, hygiene, hunger, etc.), social company, or anything like that. There is no rhyme or reason, just randomness.
It's a shame that The Sims 3 did a lot to improve the series, but also did a lot to screw it again.