This was clearly someone's first hack, and has all of the trademarks of one. Cement block architecture and cutoff in some places, "spammy" terrain made up of a single object type, levels that don't even try to hide the fact they were edited off of the original SMW levels (some of which are blatant edits of the originals), unedited title screen, and an overworld that is largely unedited after a certain point...and the hack even continues going into the unedited, original SMW levels after the second world.
What is changed is nothing impressive to be honest. The ghost house is just plain unfair, by starting the player inside a few Boo Circles that they must fall through--if you're small you will probably die a lot right off the bat. Clearing the normal exit of the secret water traps you on an overworld tile leading to the endless bonus game. The rest of the hack just isn't remarkable enough to really warrant going into with everything else I've described,