Living rooms fail most often on scale, not style. Leave 30 to 36 inches of clear walking path around the main seating group, and 14 to 18 inches between the front edge of a sofa and a coffee table — close enough to reach a drink, far enough to walk past comfortably. A rug under a seating group should be large enough that at least the front legs of every major piece sit on it; a rug that only the coffee table touches reads as undersized no matter how nice it is.
Matching three- and five-piece "living room sets" are the easiest thing for a store to sell and often the least interesting way to furnish a room — a sofa, an armchair and a coffee table from three different lines, in complementary but not identical tones, usually looks more considered than a boxed set. A retailer who will happily sell pieces individually, and tell you honestly whether two items will actually sit well together, is doing more for you than one who only shows coordinated groupings.
Beyond seating, a living room's case goods — media consoles, bookcases, side tables — carry real weight limits and cord-management details that don't show up in a photo. Ask a console's actual shelf load rating if you're putting heavier electronics on it, and whether cord channels are built in or an afterthought. Lighting is furniture too: a single overhead fixture flattens a room, where two or three lamps at different heights do the job a designer would do for free.
The retailers worth returning to are the ones whose sales floor is arranged as rooms rather than rows — a sign they think about how furniture lives together, not just how it sells individually.