I've always thought it would be interesting to get the STDEV of the distribution of WPCT of teams across various leagues. In football and basketball, you're right, winning percents of .700 or even .800 are not uncommon for the best teams in the league, and conversely .300 down to .200 isn't that uncommon either, whereas it's an extraordinary year in baseball when even a single team is outside of .350-.650.
What's more interesting to me about that factoid is why that is. My hypothesis would be that a team's superiority or inferiority to another team is more "repeatable" in other sports (well, at least MLB/NHL vs NFL/NBA), in that a top-5 team in the league will beat a bottom-5 team in the league 9 times out of 10 in basketball (And probably 95 times out of 100 in football), whereas it's rare that a season series for two teams in the same division is more lopsided than 13-5 (72%-28%) either way. A basketball or football team is more likely, it seems, to have better talent win out over the course of a single game - whereas it takes a 162-game season in baseball for the cream to rise to the top.
I have to assume that's either due to (A) a significant talent disparity between the best and worst coaching of NFL/NBA teams, (B) more discrete events (possessions/plays) over which to measure a team's skill, © better achievement of parity in MLB (doubtful considering the STDEV of WPCT probably hasn't changed much from the game's early days), or (D) a narrower gap in MLB/NHL between the merely-average players at the major league level vs the stars, and a greater gap between average/stars in the NFL/NBA.
Maybe it's a discussion perhaps better suited to General Sports, but knowledge of that disparity (assuming it exists) probably should affect how one feels about season-long performance - i.e., there's a lot more noise in baseball's results, so don't take a game or two difference in the season as seriously, or something along those lines.