Some mild additions such as some new weapons, and a day / night cycle during levels have been added while slowdown has been greatly reduced and enemies locations have been mixed up a bit but it's not quiet clear how these minor enhancement could change to meg count from the previous 362 to 506!
On a side note, Metal Slug X comes in a cool clear cartridge.
Bottom line: smoother gameplay compared to Metal Slug 2. More a revision than a true sequel 9/10
from retro to modern gaming 
