I agree with what many have already expressed.
The Model 3 is a great car.
Immediate (not subtle) noticeable differences between either of our 2012 and 2013 Model S and Model 3 with 18" aeros.
1- Road Noise, even at 30mph, there is more road and/or tire noise throughout the cabin compared to our previous S's with air and 21" wheels.
2- Suspension/Handling- The 3 (with 18" Michelin MXM4 / aero rims) is taught, nimble, and precise, however, this comes with a more firm, jarring and bumpy ride compared to an S with air and 21's. I like it. I wish my Model P85 was as firm and responsive. I find the air suspension while great for touring, is a very floaty in the Model S.
3- Motor whine - Compared to our RWD Model S's, the 3 has a more immediate motor whine when you accelerate or even if you're just cruising.
4- Rear camera quality/visibility. At night it's almost useless in my driveway backing out. The Model S, while a bit grainy still resolves shadow detail at night.
Differences that are more YMMV..
1- Screen - this has been the most difficult to get used to, I oddly find myself looking around in front of me when all the info is in the center. I miss the energy graphs, trip meters, etc that is on the Model S instrument gauge cluster. The 3 has trip meters but you have to "swipe" to see them.
The screen quality seems better (sharper) and the UX is more responsive than our S, however, I'd personally prefer the angle and layout of the Model S screen. There's no dual layout like in the S. You have maps and then have to hit the music button and/or swipe up or click the up arrow icon to get the music to full screen. Even though the S is less sharp and slower, I'd take the S over 3.
The side to side off angle viewing angle is poor, the screen is noticeably less contrasty and dimmer if you're short like me and sit closer to the steering wheel. I find google maps looks very dark and hard to see but if I sit in the center of the rear seats and look at the screen it's brighter and more or less normal. I wish there was a way to control the gamma or brightness of the maps to compensate for poor off angle viewing.
Glare, I don't have pano roofs on my Model S's but there's quite a bit of glare on the far right hand side the Model 3 screen. The Model 3 screen is glossy compared to the matte finish of the S.
2- Trunk opening/closing. We were spoiled by the power hatch on the Model S. To close the Model 3 trunk takes a lot more force to lock. The hydraulic pistons that raises the trunk seem too stiff and makes closing the trunk more difficult. My previous Lexus if you had the trunk say more than halfway down the momentum and gravity would allow to close on it's own, the model 3 will basically stay static unless you force it down.
3- Seats. They are definitely smaller as well as reasonably comfortable (still need more seat time) but a bit "foamy". I can't describe it, they feel soft yet hard at the same time. The middle back seat feels like there's something beneath the seat. I was sitting in the back center leaned forward and it felt uncomfortable while our Delivery specialist was going over the car.
4- Greenhouse effect. Coming from solid roof Model S's, we don't have the option for a non glass roof in the 3. The inside temps in a SoCal winter of 70+ degrees the inside of the car gets quite warm, warm enough to want to turn the AC on. Tint will be needed for hot, sunny locations.