The major difference is diastatic power. In a mash, base grains provide enzymes to convert unfermentable starches to fermentable sugar. When steeping, the primary goal is to get flavor and color components from specialty grains.
For example, it is entirely possible to steep roasted barley by itself and get a lot of flavor and color. There will be a small amount of starch and sugar content as byproducts, but that content will not have a huge impact on specific gravity, though the roasted barley's other contributions will be quite evident and desirable.
Crystal malts behave differently than highly roasted malts when steeping. The added fermentable and unfermentable sugar content is significant, since those grains actually contain quite a bit of already converted sugar, making them perfect for steeping. Whether steeped or mashed, crystal malt will have an impact on specific gravity.
On the other hand, steeping unmalted grain (raw wheat, flaked barley, oats, etc) without a base grain present will chiefly contribute starch and protein content, most of which is likely undesirable. While it may have a significant impact on specific gravity, it will also quite likely have a negative impact overall.
"Steeping" base grains (2-row, wheat malt, etc) may actually constitute a partial mash. However, steeping is usually done with a very high water to grain ratio at imprecise temperatures in comparison to true mashing. High pH, an inefficient lauter/sparge process, and imprecise temperature management lead to extreme inefficiencies in conversion and extraction. As a result, this pseudo-mash will yield unpredictable and likely undesirable results.
