I only have a DSi with Wild World, but one of my friends has New Leaf and she's been obsessed with it. I did manage to pry her 3DS from her long enough to try it out. She had Streetpass turned on when we were at a convention and it gathered info for about 30-40 people's houses which you could then go and visit. I thought that was a pretty cool feature, but I don't think I'd be going out to buy a 3DS just for this game or anything. I'm not too keen on the 3D feature and that was kind of the point of the 3DS
It was the point as a marketing gimmick. Actual gameplay applications with it are close to nil, and as always it's the games that should sell the system... which, admittedly, are still fairly sparse. But Mario Kart 7, Fire Emblem, Animal Crossing, Luigi's Mansion, and Monster Hunter are all really solid if those are your thing. There's also a few good original eShop titles and a bunch of good NES and Game Boy Virtual Console releases.
Although for anyone considering getting an original model 3DS and not the obscenely over-large XL, get a screen protector.
You'll be taking a flip of the coin as to whether your unit's upper screen will get scratched up simply by being closed over the lower screen. It's just as stupidly arbitrary a design defect as the cracked hinges on the DS Lite.
What's different (compared to Let's Go to the City)?
In a word, "enough".
If you ever played the original Animal Crossing and then still managed to get into Wild World, this feels like about the same amount of advancement. Just a ton of little tweaks and additions. I managed to get a couple years on and off out of the first two games. Believe me, I was pissed with City too. Traded it back in after 3 days.
This time you get to be mayor, which enables you with such privileges as enacting an ordinance (like the Night Owl ordinance, making stores stay open later, thank God), and funding public works projects, mostly decorative but includes stuff like additional bridges and the Dream Suite which lets you visit friends' towns in a "dream" even if they're not online. You can also fund the Reset Centre to the tune of about 350k bells, because Nintendo got enough complaints from soccer moms with traumatized children that you now have to FUND Resetti to have him come chew you out. You can abuse your mayoral authority to evict the townsfolk you don't like. (The ones who intend to move out also give you a few days' notice and will bring it up with you anytime you talk to them, so you're less likely to lose someone you like.)
You can choose where to put your house, and you don't share it with everyone else on the same save file. You can customize it with different doors, exteriors, roofs, fences, pavements, and mailboxes (out of an admittedly limited selection). Once you've maxed out your main room, you actually get to pick what room (including the basement) you want added next.
Weeds take longer to grow en masse. Flowers actually sparkle to let you know when they've been watered. The ring pattern in a fresh tree stump has a possibility of containing a rare design to collect. There are new fruits now, and very rare, Perfect Fruits which can be planted and sell for a massive deal more; but Perfect Fruit trees can only be harvested a limited number of times before withering. Custom pattern design has more options, and you can actually custom-pick your palette from the 96 or so colours available. You can send and receive them as QR codes. Many furniture items can now be reupholstered or otherwise given different colour schemes. The Island has been brought back for the first time since the original game; it's now a multiplayer minigame hub. Going online doesn't cause all your villagers to retreat into their houses and all festivals to cease to exist. Bees are marginally less bullshit to actually catch. It'll take a couple weeks to unlock, but KK Slider DJs at a club every night of the week now where you can go to listen to a 6-hour long remix medley of every one of his songs, with his usual live performances on Saturdays including about an hour's worth of new songs.
No more custom constellations, though. And Blathers has lost all of his flavor text. But besides that, I'm impressed.
There was a signature here. It's gone now.