I'll try to list all the significant differences I can remember from the SMS version... (other than the amount of lemmings)
I've bolded the levels that are entirely new, although Mayhem 17 and 21 are debatable as to whether they're new or not... (Mayhem 17 is identical to Mayhem 21 of the PC version in terms of layout, but the skills and solution are entirely different, whereas Mayhem 21 is completely identical to the PC version's Taxing 21, except for the standard stuff for SMS version(no more than 20 lemmings, different graphics, etc) and the name)
Fun11 - Keep Your Hair On Mr Lemming (no builders, IIRC no miners or diggers)
13 - We All Fall Down (release rate 20)
14 - Origins and Lemmings (no floaters)
16 - Don't do anything too hasty (requires landing on the first spike before building up to the exit)
17 - Easy When You Know How (only one trap, not four, no floaters)
20 - Sega One (entirely new level)22 - Go For It (entirely new level)23 - I've Lost That Lemming Feeling (no hidden exit, less flames that kill the lemmings)
24 - Konbanwa Lemming-San (no trap, even though you can still see it it does nothing)
26 - Sega Two (entirely new level)27 - Let's be careful out there (just one miner and the level is solved)
Tricky2 - We All Fall Down (release rate 40)
10 - There's a lot of them about (view starts on one of the entrances instead of the middle... or is it in the middle? opposite of the PC version anyway)
14 - Menace (entirely new level, even though the name is the same)16 - Luvly Jubly (due to an extra steel block in the Sega version, you have two bashers)
19 - Postcard from Lemmingland (you have a blocker and a bomber and you can lose one lemming, even though 100% is still possible without using either)
21 - Sixes Not! (entirely new level, kind of... it's still three 6s in the fire style, but they're made of smooth blocks instead of rocks, you only have 20 of each skill and 20 lemmings, still requires 66% though (in practice, since you can't acheive 66% and it won't pass you for 65%, it requires 70%)23 - From The Boundary Line (release rate is 50, not 99)
25 - Sega Three (entirely new level)Taxing2 - Watch Out, There's Traps About (there's only one trap, the one in the ground to the right of the entrances, the other one of these traps and the squasher are not present)
3 - Heaven Can Wait We Hope (see Fun 16, also, no brackets in the title
)
4 - Lend A Helping Hand (no traps)
6 - Compression Method 1 (see Fun 17)
11 - The Ascending Pillar Scenario (although it isn't DIFFERENT, the time limit is much more challenging in this version)
13 - Upside Down World (although the layout is basically the same, the level is a lot smoother and lacks a trap, I think the skills differ slightly too)
15 - What An Awesome Level (entirely new level, even though the name is the same)17 - X Marks The Spot (see Fun 23)
21 - Sega Four (entirely new level)22 - Come On Over To My Place (see Fun 27)
23 - King Of The Castle (requires 100%)
24 - Take A Running Jump (can't get up to where the "jump" is(except by building up to it), you have two diggers and a basher as well as builders, the fall near the start is unsurvivable)
28 - Poor Wee Creatures (no traps)
30 - Sega Five (entirely new level)(Yes, Taxing doesn't have a We All Fall Down in the sega version)
Mayhem1 - Steel Works (the first fall kills the lemmings as well as the second, unlike in *some* PC versions, however you have enough floaters for every lemming)
2 - The Boiler Room (you have enough room to build up and past the flame, rather than having to bomb through)
3 - It's Hero Time (see Fun 24)
4 - The Crossroads (requires 100%)
9 - Sega One (entirely new level)10 - Pillars Of Hercules (not new, but notable for being the second level on the Sega version to require a % not ending in 0 or 5, like Tricky 21 it requires 66%, so in practice the requirement is 70%)
11 - We All Fall Down (release rate 80)
12 - Sega Two (entirely new level)
13 - Sega Three (entirely new level)14 - Pea Soup (instead of croutons, there are blocks above the soup)
15 - The Fast Food Kitchen (see Tricky 10)
16 - Just A Minute (you have two minutes, and the little platform is too low to bash to the exit from)
17 - Stepping Stones (entirely new level despite having the same name, it's a harder version of Tricky 12, it's different than Mayhem 21 in the PC version though)
18 - Sega Four (entirely new level)20 - No Added Colours Or Lemmings (you have three builders, it takes two to cross the gap)
21 - With A Twist Of Lemming Please (not exactly an entirely new level, it's identical to Taxing 21 from the PC version, even though it has the same name as the PC version's Mayhem 21)
22 - Nearly There (entirely new level)23 - Going Up (no trap)
27 - Just A Minute Part Two (you have two minutes, also the little platform isn't missing, but again it's too low - in effect, the two Just A Minutes are identical in the SMS version, apart from extra skills that have no relevance to the solution)
28 - Mind The Step (possibly the trap doesn't work, as with Fun 24 / Mayhem 3, I've never actually let the lemming walk into it so I can't say for sure)
29 - Sega Five (entirely new level)30 - Rendezvous At The Mountain (no traps)
