What would the world be like if everyone had the same personality?

That probably depends on what personality it was. It could be a lot more giving, loving, charitable, and generous. Or, it could be more petty, mean, and cruel. We'd probably understand each other a lot better.

Either way, if everyone had the same personality it might get boring, even if it was a great one... and it might get old if none of us could change and improve.