There goes the neighborhood.
The main one, of course, is the new game space. No more impenetrable "fog of war" beyond the border of your lot. Your sim can walk out of your backyard and keep walking until he hits downtown. The downtown, and really the entire game, is now a single whole, not a bunch of lots floating in space. The downtown area boasts a movie theater, a supermarket, a couple of food joints, a spa, a city hall, a school, an office building, an art gallery, a bookshop and library, a couple of pools and parks. There are also a stadium, a science lab, an army base, a cemetery and parks on the outskirts. This sounds like there are a lot of things to do, and there are, but unfortunately most of them you won't see. You can walk around the library and art gallery, but strangely, watching a movie, a concert or a game consists of watch a little logo and a bar under it fill with green. Same goes for eating at a restaurant, going to class or work, or "touring" places like science lab and army base. Fun times.
The number of sim interaction options has been increased, with a lot of fun new and even contextual "emotes" added, but public and hot tub woohoos have been removed for some reason and woohooing now gets old real quick. In fact, I was pretty much bored stiff playing my single flirty young adult writer, and he got married as a result. Speaking of the public woohoo, there is no clothing store in the game for some reason, and your sims will pretty much look the same all the time.
Now as I said, marriage does make the game better, as you have more on your hands. And when your wife gives birth to twins, on top of the two kids you already had, the insanity of juggling family and work is extremely entertaining. Building a house and then adding to it as your family gets bigger is also a lot of fun. The careers are not so much (though granted both of my adults are in journalism and the children are too young to work still). The career opportunities are fun at first, but there seem to be no more than a dozen different ones per profession, and going to the city hall to get an exclusive story on a brawl or something gets less and less exciting every time you do it. Owning properties in the city is a neat new option, and you can potentially own the whole town eventually, though there isn't much you can do with them aside from making money. Yes, you can rename businesses and fire sims, but that's fun for about 4 seconds.
The game looks better than Sims 2, but most of the improvements come from landscapes and exterior views. Character models, though I'm sure they must have been worked on, look the same to me personally. The create a sim system has been improved, but the only difference that stood out was the existence of new character traits you can assign your sims, modding their personalities and potentially the world's response to them. You can also create fat and skinny sims now, which is a great addition, but you still can't tweak the height. Maybe in Sims 4.
The sandboxing remains the main part of the game, and, judging from sales figures, a pretty popular part. On me, the Sims 3 produced the effect similar to Sims 2. I played and had a lot of fun with it, but never dropped money on any expansions. I anticipate doing the same this time around.