It was only questioned in Boston, where he was prematurely annointed Capitan and had expectations set way too high for him at an early age. And his laid-back personality was also at odds with the Boston media's desire to make him the next Orr. In SJ, his personality is a perfect fit - for both the team and the city - and don't forget that he's no longer under the pressure of wearing the C.
re: playoff performance.... If you look at his boston stats and remove the 97-98 playoffs when he was an 18yr old rookie and the 04-05 playoffs when it was later revealed that he was playing with cracked ribs and torn cartilage in his chest, he was 6-12-18 in 22 games (.818 ppg). He was a .852 ppg guy in the reg season for boston. Granted, not exactly setting the world on fire, but that's a negligible dropoff when you consider that playoff scoring is always less than reg. season.
In the 140 games since coming to SJ, he's 42-164-206 (1.471 ppg). so yeah, regardless of what happened in Boston, I think he likes his new situation. Detroit will be playing the SJ Thornton, after all.
And I agree with you... Marleau should be Detroit's biggest worry this series, offensively at least.