Well, I know that only two things were slow on my Amiga: AB3D II and Extreme Racing. NatAmi has more then enough power for those games. I would say several times more than enough power.
Soo one more thing is the amount of chip ram. Me and my bro always try to make marathon cartoons and run out of chip ram. So maybe now we can really let loose.
Easy connection to modern screens is also a big plus.
I know it will be much faster in rendering, but for my simple scenes in Maxon A1200 with fast ram is also enough
That's just off the top of my head for now...