As much of this work is moving into the realm of the art director and art team, the level designers remain the digital architects and they will still be responsible for much of the look and feel of the levels. Therefore, if a project calls for an accurate Roman Empire then everyone had better be doing his or her homework. Having a shared directory of R and D images on a server as well as an art bible that is referred to by all designers and artists will contribute to a more consistent look and feel. More often than not he'll discover little details that make no sense, such as structurally impossible architecture that seems out of place or ice beasts near lava pits. The users may not notice these details on the conscious level but will sure feel it on a subconscious level which will affect the overall game experience negatively.
Good research lends itself to good planning.
Some designers simply sit down and build while others carefully plan every nook and cranny of the game. The best designs are the ones that are a combination of careful on-paper planning and improvisation. If the designers are working with a technology that can push 100 million polys then they're going to try to make it look like it can push 3 times that. Although much of the framerate issue falls upon the programmers, with optimizations and level of detail technology, it is extremely important that designers have hardcoded guidelines for framerates, detail levels, and RAM usage. The lead level designer should be the one responsible for enforcing hardcoded design limitations.