Solid wood tabletops move with humidity — they expand slightly in summer and contract in winter — so a well-built solid-wood table is engineered with breadboard ends or a floating-panel construction that allows for that movement without cracking. Veneer over engineered wood (plywood or MDF core) is dimensionally more stable and often flatter and less expensive, but is vulnerable to chipping at edges and can't be refinished more than once or twice. Ask directly which construction you're looking at; "solid wood" on a tag sometimes means solid wood legs with a veneer top.
A catalyzed conversion varnish or a UV-cured finish resists water rings, heat and scratching far better than a basic lacquer or oil finish — ask what finish is used, since this is the detail that determines whether the table still looks new after five years of dinner plates and reading lamps set down without a coaster.
Extension mechanisms vary in quality: a butterfly leaf that folds and stores inside the table is the most convenient day to day; a separate leaf stored elsewhere is more work but often a simpler, more durable mechanism. Standard seating math allows roughly 24 inches of table edge per person; a table needs about 36 inches of clearance behind each chair for someone to push back and stand comfortably.
Chairs bought separately from a table should be checked against seat height (typically 18 inches) and table clearance (10 to 12 inches between seat and tabletop underside) before ordering — a store willing to do that math with you, rather than just selling a matching set, is worth the extra time.