Although their games are mass mass market, they still feel opinionated. Their designs have a certain Nintendo-ness to them that's persisted for decades in almost all their games. Even when they partner up with a third party their design shows through.

If they had extensive playtesting early on I would think their uniqueness would be eroded. It obviously hasn't. Most recently Nintendo set the bar for what an open world game should be like with their take. Not just in quality but by applying their method they elevated the genre.