I'd add the technical advancement of something like Kid Chaos or Elfmania.
But as a very last item.
I'd say the simplicity of RI goes in the field of playability, really.
And today, I rather have a fast arcade game than something as long-lasting as Frontier. It should have, though, immense replay value, through the unlocking of stuff, or whatever.
I go back to Sonic 2 on the Megadrive all the time, even after many years of its release and my first play at it, and it's not as complex as Frontier.