As someone who was actually following one of the biggest negative threads about The Sims 4 up to its release on another forum, I was very pleasantly surprised by how much I like it. There are some major differences that are frustrating (for me, no Create-A-Style and the loading screens that limit your ability to see what the rest of the world is doing) but it was not enough to make me dislike the game. Manage Worlds isn't as nice as Edit Town, either, and I dislike that you're forced to enter a new game through the CAS menu and placing a sim, and can't just edit an empty town or play a premade off the bat. But there's nothing stopping you from making a throwaway sim, and then hitting pause and going into Manage Worlds, where you can then delete the throwaway household and play a premade or edit a lot. You can then save that as a base file for new games if you want, as long as you remember to "save as" when you do it.
I love how fast Sims 4 runs. I have overkill stats for Sims 4 because my laptop is a gaming laptop that's been upgraded with a solid state hard drive to run Sims 3 on its highest settings with all expansions, stuff packs, and a bucket load of store content and a good amount of mods and CC. So since it can handle that, it runs Sims 4 amazingly well.
Despite the loading screens, the live areas on Sims 4 are very much alive. There are always sims roaming about to interact with. Sometimes I find there are too many on public lots, and they butt into your conversations, but on your home lot it's not bad if you don't mind ignoring people at the door. (Which sometimes you have to do in Sims 3, too.)
The build mode wasn't as intuitive to me as it was billed, and it took a little getting used to, but once I got the hang of its controls, I love it. It's so nice to be able to actually move rooms and roofs and such without having to delete and rebuild. I miss my basements, but I wouldn't be surprised if we get them eventually.
Multitasking is amazing, and I found I really missed it and the easier group conversations when playing Sims 3 for a bit yesterday. It's a real chore to get 4 sims to interact together in Sims 4 requiring a careful queuing of who's talking to who next, but in Sims 4 it happens naturally. It's also nice that your sims can eat or cook or use the computer and actually talk to someone else, without having to cancel it and wait for them to stand up or put down what they're doing to answer a simple chat interaction.