I know about the Poison Mushroom and the difficulty of the sequel, but what is the main other difference?
Super mario bros: The lost levels was, in japan, Super Mario Bros 2;
But it was deemed as 'Too hard' for US audiences. So Nintendo took another game named Doki Doki Panic, reskinned it with Mario, and thats Mario 2 US.
The Japan Mario Bros 2 was then released in the US a few years later, but with Mario Bros 2 already a game, they named it The Lost Levels.
So Super Mario Bros: The lost levels is the direct sequel, with different levels and some different mechanics, to Super Mario Bros.