That's funny, I've noticed just the opposite. New brewers (at least ones I've sampled beers from) , especially ones that start as extract/steeping grains brewers, often have overly sweet , underattenuated beers due to the lower fermentability of many extracts. And making things worse, a lot of them use way, way too much crystal. But to answer your question, to build body in an AG beer, a higher mash temp (154-160F) along with use of wheat, rye, or flaked barley would help, as would choosing a less attenuative yeast strain. As for being flavorless, that's a recipe issue. Gotta experiment with malts and hops.