Mario games consistently have tight controls, brilliant level design and endearing aesthetics.
I also think Nintendo managed Mario well when he could have fallen off. Back in the NES and SNES days Mario was like Mickey Mouse in terms of cultural relevance. Many companies would have run such a phenom into the ground, but Nintendo showed remarkable restraint.
Then Mario 64 landed and shook the whole industry. How many series can redefine an entire industry twice? Yet, again Nintendo held back and no 64-2 happened. The Gamecube saw one entry in Sunshine and really it was not until the NSMB sensation that character was threaten by main line over exposure.
Nintendo have managed mostly successfully by splitting the series with 2D and 3D entries to appease different groups. At the core though they are unparalleled as plat formers with the Galaxy games in particularly being true classics.
I think Mario will see highs and lows, but as long as Nintendo manage him I fully expect to see him in quality major releases for years to come.
