Deathnote is just slightly borderline, a little more than usual Shounen but not necessarily Seinen yet. As a Seinen it would be pretty Shounen actually, though it would perhaps be a possible rating.
Shigurui is a Seinen in every relation imo, exept for the official demographics which is stated as "Shounen" due to its appearance in Champion Red.
-Massively explicit violence/gore, lots of blood, precise and detailled crossections through cut bodies
-Explicit display of sex in partially pretty sick backgrounds (almost incest etc.)
-A cruel and disgusting world (madness, bloodlust, incontinence, crippling, psychic cruelity, torture, barbaric punishments)
-The storytelling and characters aren't superficial. It's sometimes hard to understand their motives and ways (although possible), it's not a flat type of story,
-A mainly realistic world and precise display of japanese feudal system. Only "exceeding reality" in details (sometimes fighters are "a little stronger" than humanly possible for example and swords perhaps cut a little too good, but the basic scenario is extraordinarily realistic especially for a samurai scenario!)
So, graphics fit for adults only, a most serious world/scenario, storytelling and characters easily deep enough to compare to very most Seinen. This is Seinen every way I look at it, as it's not suited for children in any way and definitly no Josei.