At this point, I think anyone that's wanted to see the glasses free 3D screen of the 3DS has seen it, so I'll focus a bit more on some less obvious things.
But I'll start with my own experience of 3D. I don't usually use it with the 3D Depth Slider all the way up, usually at 50% to 70%, and mostly just with PilotWings Resort where I think it's valuable to the experience, and the camera functionality where it's necessary to capture a 3D photo. With the 3D Depth Slider at 0% to about 15%, I can tell that the parallax barrier is on, but I can't say that I really notice 3D, even in a game like PilotWings Resort where you can have a huge spacial distance between the closest object on screen and the farthest. I have no trouble seeing 3D, and 3D doesn't give me headaches at all, which isn't unexpected. It's always been easy for me to see "Magic Eye" 3D images and I've seen many 3D movies without headaches.
The "sweet spot," the ideal zone in which to view the 3DS' top screen's 3D is quite small. I heard many hands on reports that didn't quite agree on this; for every four that would say you only get about a centimeter to move your head left and right, there would be one that would say you have about an inch and a half. They're both right, but when you go beyond that centimeter or so range, you start noticing the parallax barrier darkening the image, but the image is still obviously 3D. As I understand the technology, having a "sweet spot" that small is the best you can do when you get the columns of pixels down to as small as they are on the 3DS. It took a few hours, but I don't have any trouble now keeping the 3DS at an angle suitable for viewing 3D. For me eyes, I can move the 3DS about a foot backwards and forwards to my face before I get distortion, so there is no adjustment with holding the 3DS at a different distance than I would with a GBA SP, DS lite, or any handheld gaming device.
One thing I didn't catch being mentioned about the 3DS' 3D screen is that the 3D actually slots up clearly at two additional angles. As you angle the 3DS, you'll see the image darken and clear multiple times; straight on, is the "sweet spot", a bit to the left is a view where your right eye sees only the image meant for your left eye, and your left eye sees only the image meant for your right eye, giving a reverse 3D image. Speaking of reverse 3D, I'm actually baffled as to why you don't get reverse 3D when you view the 3DS upside down. It seems to me like the image that was meant for your right eye would now be reaching your left eye and vise versa and the image would be 3D, but not so. Very interesting and I hope some devs take the opportunity to have games with the player (or a second player) viewing the 3DS upside down. A bit to the left of the reverse 3D is one of the other spots to view 3D, and the save for tiling to the right. Beyond that, and you begin seeing the left and right eye images with both eyes so the effect doesn' work. So this idea that you can't share the 3D with more than one person at the same time is technically very incorrect.
I touched upon it in a previous section of the impressions, but the 3DS screens have a smaller viewing angle than DS lite screens or DSi XL screens. In the end it's enough for normal playing so it's not a flaw I would name of the system, but it is something I suspect will be improved in the next iteration of the 3DS, if only to be another item on a comparison list.
On paper, the pixel resolution of the 3DS didn't get that big of a bump from the DS systems. I actually ran some numbers one night just to see what kind of percentage increase the resolution got from the Game Boy Color to Game Boy Advance, Game Boy Advance to DS (one screen), and now DS/DSi to 3DS (bottom screen vs. bottom screen). Like I usually do, I was scribbling out the results on an open session of PictoChat so I don't have the numbers anymore, but the bump from DS to 3DS was par for the course of Nintendo handhelds. I said that to say that in practice, the resolution difference from DS to 3DS seems bigger than ever, especially when viewed with 3D on. I've seen two or three people make the claim that the 3DS' 3D screen looks better than HD, and I have to say, that sentiment has some merits. I don't believe you can fully validate that idea, but the mental perception of each is very comparable.
Has anyone else noticed anything interesting about the screens or 3D that they feel is worth mentioning? Maybe something you noticed because of another 3DS launch title?