Playing DS games on your 3DS results in a loss of quality
While some people will dismiss this as unimportant, I think it’s fair to say that I should at least put it out there before making the assumption that no one wants to hear it. According to some tests by Destructoid editor Nick Chester, the 3DS actually displays regular DS games in poorer quality. Check out these photos he took:
As you can see, there’s a clear difference between the color intensity and the blurriness of the two screen photos. The one that seems a bit blurrier and less vibrant is indeed being played on a 3DS, vs. the clearer one on a DSi.
Of course, you can play DS games in their native resolution which cleans up the image a bit, but it also shrinks the size way down. You can see in that third screen that Pokemon takes up very little of the already small 3DS screen.