Not all the features of the original game have been implemented, and one or two things have been changed.
Graphics icons are used instead of characters to represent the levels on screen. On some levels (e.g. 7, 14, 24, 31) the original screen had characters which spelled out readable words. These are no longer readable.
Baby monsters are not allowed to move on top of walls, and therefore they can't move outside the playing area (as happens in the original level 41, which has been changed slightly).
The original small window through which the level was viewed hasn't been implemented, only the full-screen view.
(Personally, I always preferred the full-screen view, because the small window just meant you boringly had to draw your own map of the level, detracting from the puzzle solving. Besides, the puzzles are hard enough as it is, especially for people with modern short attention spans.)
There is no high score table.
There is no password system for going between levels.
There is no level editor.
The arrow keys and w,a,s,d are supported as well as the original h,j,k,l, but there is no way to redefine the keys.
Bombs and thingies haven't been implemented - but neither of them appear in any of the standard 60 levels.