I see the years of the above albums as times when Metallica was exploring the boundaries of metal and rock. Think about it for a second and it’ll actually make a lot of sense. Until It Sleeps could easily be considered “gothic metal”. Mama Said is country. St. Anger could almost be seen as nu-metal. Do I like everything put out during that time? Nope, can’t say that I do. But a few gems definitely stick out and I’ll enjoy them quite happily.
So what does the future for Metallica hold? Well, the band has stated that there will be a new album in the near future, even possibly this year (but I’m not holding my breath). Lars has stated that the songs will be shorter and more to the point than the tracks on Death Magnetic. Kirk said that “…if Death Magnetic was a logical successor to [1988's] …And Justice For All, the next album will be a heavier Black Album.” [source]