I think when people refer to "mature" slice of life to you they probably mean "not something that has to do with a high-school and/or a club and all the usual comedy elements". Since, for example, Honey&Clover is in a college, not a high-school.
Stuff like Aria, or Yokohama Kaidashi Kikou. Or the recent Gin no Saji.
But tbh while not maybe being as "mature", stuff like Lucky Star or Yuru Yuri is really good SoL.