I haven't heard "Death Magnetic" but there are at least two obvious problems with it.
One is that Metallica, like other bands on that scale, is more of a phenomenon/concept/brand than simply another band, so most of the time one dislikes the album because it's Metallica or likes the album because it's Metallica, regardless of its actual merits and failures. Fans will pick it up and those who don't like the idea so much to begin with, would need a fucking phenomenal album to be convinced.
The second problem is that when "return to form" is even under discussion, it's obvious that there was something seriously wrong with the dynamics or integrity or spirit of the band and when it happens, it would be better in almost every case that the band would have quit there and the members went on to play different music that is truly relevant and interesting to them today. There are some bands like Profanatica who have been around for a very long time and away from the scene but no-one is surprised that they returned with a good album, because there was never a reason to suspect anything else.
Justin Broadrick's Godflesh had become quite boring but when he took the steps to move into another style he made more exciting music again, I mean Jesu. For normal metalheads it might seem Beherit's Holocausto never "returned to form" but I think the Suuri Shamaani album is a masterpiece.
In a context closer to Metallica, Judas Priest did "return to form" with "Painkiller" after a couple of lackluster commercial-oriented efforts. It's a very different case to Metallica's 15-20 years of utter drama and uselessness though. Iron Maiden has also produced a few strong albums after failures in between. But that's maybe because they always had good musical ideas and they can still practice some of them even if they lack the original rebellion spirit... Metallica was never "good music" as such but it had the anger and spirit back in the early days.