In my experience, the differences between 720p, 1080i, and 1080p are not too noticeable on most screens, especially if the screen is under 65".
However, lower resolutions are quite noticeable on 50"+ screens. That said, the game is being upscaled to 720p anyway, so it will still be hard to discern the difference. People are just throwing a fit because it isn't natively 720p.