Totally reviving an old thread, but...
NES: The levels are much harder due to the mechanics of the game. Some will say they did the best they could, but look at how good the Sega Master System version is. Anyway, the levels are squeezed into a smaller space (look at Use Bashers This Time, it's tiny). Lemmings build much steeper ladders, but that's OK because the levels are smaller. When Lemmings bash, whole blocks of terrain disappear, not just pixels-per-bash. When Lemmings dig, same thing. When Lemmings mine, about 3 or 4 blocks of diagonal terrain disappears even before the Lemming has got there yet. This leads to the interesting idea that Lemmings will pass a miner and fall through the ground while he's still mining. Sometimes Lemmings won't do a job because they don't have room to, but you still lose the skill. For an example of how bad this game was, just play Just Dig. If one Lemming digs, he will live. If all the rest fall down his hole, they will all die because someone turned gravity on too high.
GB: Same thing as above, but black and white.
GBC: This version contained Oh No! More Lemmings. Alot of the levels are missing, and none of the music is the same as any other version. It's superior to the previous two versions in that gravity has finally been fixed so Lemmings don't die from innocuous leaps.
Macintosh: The funniest thing is the censorship. The devil exit has been replaced with what looks like a sort of melted birthday cake with eyes and buckteeth. Also, the levels are squashed in horizontally similar to the Sega Mega Drive version.
Sega Mega Drive: The terrain are all different colours, and as mentioned above have different levels replacing older ones.
Sorry to revive old threads, but information helps many.