Those of us who like the game usually like the sims, which seem more real as opposed to the TS3 sims being more robotic. They have many more social interactions, and I've gotten so used to their multi-tasking that I don't really notice it anymore. The children are cuter and have more to do, possibly to make up for there not being toddlers. The game loads much faster for me -- under 5 minutes as opposed to 20 minutes or more for TS3. Building is easier, but there's less furniture to choose from and no create-a-style.
Speaking for myself, I've been very pleased with TS4. The one really bad bug I had (deformed babies) was fixed within the first week, and they've added ghosts and pools in subsequent patches.