What is the main general difference between Mitosis and Meiosis?
If the question is about the one and only most important difference between mitosis and meiosis, then the answer "meiosis reduces ploidy" is probably correct. But if the list of important differences is open, it would be critical to add that mitosis generates identical cells (identical to each other and any ancestral cells, barring rare new mutations), while meiosis generates genetic diversity by creating cells, which are all different from each other and from any ancestral cells. It does so by independent segregation of homologs and by crossingover within chromosomes.