1) The top 4 guys are, simply put, the best old guys to ever play the game. Kareem was 22 in 1969. He was putting up 10 Win Shares at age 39. Malone took 4 years to reach 10 win Shares in a season, then he ripped through 14 out of 15 years from age 26 to age 40. Stockton -- same thing, just one year difference. And Wilt (except for being injured one year) NEVER had a season where he logged under 10 WS. He was 36, leading the league in rebounding, and playing at a still very high level when he basically quit over a contract dispute.
2) Duncan finally appears unable to play at the consistent level necessary to put up 10 WS. Of the bevy of players at 11 seasons, who'd a thunk that Reggie Miller would fit in with this crew??? If you had to grade these guys on a 1-100 scale you would say 95, 100, 100, 82, 94, 97, 98, 96. Miller is top 15 on the all-time WS list, but 45th in WS/48. Analysis -- he played a lot of minutes. He logged 34+ minutes a game 12 times, tied for 15th most since 1979. Is Reggie the 15th best player since 1979? Uh, no. But he was durable and consistent.
3) Russell - at the start of his career, he was about 70% Defensive Win Shares. By the end of his career each season was over 90% Defensive win Shares. 1965-66, he had 0.3 Offensive Win Shares and over 11 DWS -- 97.436%!! And people say he was a top 5 player? Better than Wilt? Come on. There are two ends to the court. Wesley Johnson so far this year has 0.4 Offensive win Shares.
4) You also see on the list the precipitous drop off in the career of Larry Bird and Kevin Garnett once they were injured. Look'em up. Bird's games played declined, and he wasn't ever as good per minute. Same with Garnett, whose WS/48 went from .265 in 2007-08 to never reaching .200 ever again.
5) Shaq, oh Shaq. If you ever want support for the folks who say, "Shaq wasted his greatness," look at this. During Shaq's 13 years he only had 9 great seasons. When you are playing 40-50 games, you probably aren't reaching 10 WS in a season. His second year, Shaq had almost 17 Win Shares. His 4th and 5th year -- 6.9 and 8.0. Now, of the 4 sub-par years, one year was a strike year, but if you take the other 3 years and give him 7 extra WS per year, you are looking at a top 5 finish on the career WS list.