So how different is it from the original nes version ?

The same, but different. Basicially, areas have been cleaned up and look nicer, some content up to the final battle was added, cut scenes added, a map added, and hints added. Beyond that, it actually continues the story a bit after the evacuation from destroying Mother Brain.

Once you complete the game, you unlock the NES Metroid in all it's old glory, including proper slowdown when too many enemies/screen movement occurs.