To me the first order of business for a Major League General Manager is that he be a good judge of talent. Not only that but he and his scouts must know as much as they can about a prospective prospect's character. That part is harder but with thorough cross checking it can be ascertained. True, it is not an easy job but with those two qualities in your possession it does make it easier for that person.
Today with so many GM's with business backgrounds there seems to be a de-emphasis on people with talent judgment skills. I remember when I was a kid and the Brooklyn Dodgers had a GM-part owner named Branch Rickey running the show. He was a near genius in spotting talent and that is how he was able to sign, trade for or draft the like of the players he did----Snider, Campanella, Reese, Robinson, Hodges, Furillo, Newcombe, Erskine, Labine. Many of those are in the Hall of Fame and some are close to getting in. I also think it is better to have two men occupy that spot with clearly delinated duties---one in charge of player personnel, the other with finance and contracts. I hate to use them as an example since I despise them since they left Brooklyn but the Dodgers for years under the O'Malley's had two such men in that job. Buzzy Bavasi ran the personnel part of it and Fresco Thompson the business end. It seemed to work for them well in the 60's.