The main difference between the two is the art. All the artwork from the blue cover original version of the module gets replaced in the revised orange cover. Also, the maps get updates, with more color and update encounter area labels to reflect the revised labels and numbering in the text of the module. While opinions vary according to personal taste on the new art, the one complaint most people have had with the map update is a fairly glaring omission made to one of the important maps of the Taboo Island temple complex.
Here's the original map, for reference.
And the same map in the revised edition. I removed the monochrome blue background from this map for easier comparison, though you will see, the change is very obvious.
Without that little inset graphic depicting the tunnel under the north end of area 5, it appears to a lot of readers that there is no way to get to area 6 and beyond. The Description of room 5 (room 34 in the revised module key) mentions the raised platform where people in area 34A, on the raised platform, may observe the activity in the main chamber (area 3 or 32 revised), but doesn't clearly explain the tunnel leading under it to access the rooms and collapsed tunnels beyond. So, with a little bit of paint shop magic, I've created a corrected map for the revised module that includes the missing insert illustration, for your reference when using the X1 module.