What exactly do you mean by "unobtrusive"? If it's appearance only, then a DSLR is a DSLR is a DSLR. The average Joe will not know the difference between a D5100 and a D800; maybe he might notice that the D800 is bigger if he looks carefully. So if you want to do street photography and are concerned that a larger DSLR would be too obtrusive, a D5100 won't help. A mirrorless or premium compact would be your best bet in this case.
On the other hand, if just want to reduce the size and weight of the D3/D300, then go right ahead. The D5100 is very light and a dream to use.