When people discuss the all-time great RBs, usually there's Jim Brown, then a bunch of players from the 1970s and later. I'd like to add a player to the conversation: Steve Van Buren.
Just look at his numbers. In a much different era, he won rushing titles, cracked 1,000 twice, had a career 4.4 average (including a season of 5.8). He was the best offensive player on an Eagles team that won two championships. In one of those championship games, he rushed for 90+ yards and a TD. In the other, he rushed for 196 yards.
By the way, check out the Eagles' playoff record in 1947, 1948, and 1949. They played 4 playoff games and pitched 3 shutouts. The 1949 Eagles could make an argument for being one of the all-time great teams.
However, in discussions of all-time greatness, a lot of commentators refuse to go back that far. There are different checkpoints for different people: some go back to the first Super Bowl, or to Lombardi's Packers, some go back to the 1958 NFL Championship game, some go back to Otto Graham and the Browns. But you don't hear of too many players from the '20s, '30s, and '40s talked about in all-time great discussions. They get some credit, but it's talked about as if it wasn't even the same sport or league. However, the '30s and '40s featured two of the true all-time great QBs (Sammy Baugh and Sid Luckman) and a legitimate contender for all-time greatest WR (Don Hutson). It's hard to talk about people who we certainly didn't see play, and who clearly played in a league with different style and rules than the game we know and love today. But when we have discussions of the all-time great players at different positions, let's not pretend that pro football started with Johnny U and Jim Brown.