Yep more than anything else illness and playing in the dead ball era lead to Sisler being underappreciated. He was one of the last great first basemen who was not a power hitter. I guess a modern analog to him might be Mark Grace who does not have a lot of power for a modern first baseman.
Here are the career stats of Sisler and Grace, Grace through the 2003 season:
Player-----AB---R----H----HR-BB---SO---BA---OBP----SLG
Sisler--- 8267--1284--2812--102--472---327--.340--.379--.468
Grace-----8065--1179--2245--173--1074--642--.303--.383--.442
A Grace link:
http://sportsillustrated.cnn.com/baseball/mlb/players/4194/
A Sisler link:
http://www.baseball-reference.com/s/sislege01.shtml
Notice reflecting his era, Grace walked and homered a bit more but also struck out a bit more but they are similar in the two most important numbers onbase percentage and slugging percentage. And note Grace played much of his career in a ballpark that existed in Sisler's time.
As someone suggested Sisler missed the 1923 season due to an infection affecting his optic nerve and was never as good after that. Thus his best years were mostly in the dead [or dirty] ball era.
And just for the heck of it let's look at Irchiro's career numbers through 2003:
Player-----AB---R---H--HR-BB--SO---BA---OBP----SLG
Ichiro----2018--349--662--29--134--184--.328--.374--.440
which looks alot like Grace and Sisler. Ichiro's numbers came from:
http://www.baseball-reference.com/s/suzukic01.shtml