I think that it comes down to materials and craftsmanship.
Better quality frames will usually have a decal that describes the tubeing that was used. You'll have to look it up to see where the tubeing fits on the food chain.
While fancy lug cut outs are an indication of craftsmanship, I don't think that's a perfect indicator of frame quality. Many of the better steel tube sets today are designed to be optimized with tig welding. The most exotic frame that I've ever owned was fillet brazed Nivecrom with very thin wall thickness.