I own a New 3DS and even if I don't use 3D that much I can tell that the improvements were made: the face tracking works extremely well so if your eyes don't get stressed you can fully enjoy the 3D experience. Another great improvement indirectly related to the 3D are the specs for the console that make it load almost anything (except pokémon games and a few others ç_ç ) a lot faster, meaning that you can go on the internet at a decent speed and guess what ? watch 3D VIDEOS ! this is probably a feature many people will miss out because in the end it isn't that usefull but still it's pretty great, especially in this last few years YT started supporting 3D effects so you should definitely check out some of them, It's entrateining and also pretty fun ! The speed in navigation makes even the console usefull for some other major things, for example, I've recently started using my New 3DS as a Music Player when I'm at home, I just find the song I want to listen to on YT or whatever other website and i play it with no loadings (and conisdering the Italian connections it's anything but an irrilevant trait)

I don't think a n3DS purchase should be based mainly on the 3D though, because all of the new features added is definitely really nice.. Faster loading times is a lot faster, for saving, loading and running applications.

Not too many games have use for the additional buttons, but and Amiibo functionality is pretty neat

But then again, if you have  a perfectly fine 3DS, then there isn't such a big need to upgrade :p