A "Lost Levels" styled hack in that there's very little in the way of new enemies and features, but there is a steeper learning curve and a ton of secrets to find within those levels. Don't worry if difficulty hacks scare you though, many of the early levels are still easy and there is a sincere attempt at level design and variety.
This hack improves on the original version in the following ways:
1. All Koopa Kids, Bowser, and Reznor are more difficult, keeping with the tone of the hack.
2. Lag is considerably reduced.
3. Sprite memory is improved in a few stages
4. Various bugs, some minor, some major, have been either eliminated or minimized.
5. A few minor level edits to fix bugs, such as removing Yoshi in Forest of Illusion 3 and removing the final Pokey in Chocolate Island 5, and one major level edit with Yoshi's Island 3 now featuring a faster scroll.
The hack is about 95% the same, but the changes generally make things more difficult. Unlike the original, it is not a pure vanilla hack as it takes advantage of a few patches and hex edits to tweak a few things here and there. However, it still resembles a vanilla hack in all essential ways, and those few changes ultimately allow it to play much better than the original version.
This hack stops being fun around Iggy's Castle. The levels become boring, tedious slogs to play through and the secret exit to Donut Secret 1 requires a stupidly overcomplicated mess involving cloning a p-switch with a bonus room. The hack tries to be hard for the sake of being hard and completely abandons fun altogether in the process, unfortunately.
I do not recommend this hack. If you're looking for a fun challenge, look elsewhere.
I've played the first world and the first level of the second world, before I decided the quit and delete this "hack". You call it the New World, but hardly anything has changed. Very minor changes in most levels. The first part of the first castle is a very near copy of a later castle in the original Mario World. Level names are not changed. It seems like very little effort went into this hack.