I sent a bug-report to Wolfram and the case number is [CASE:3454878]
I'm on Linux with Mathematica 10.3 and I have my 3D graphics output set to the highest "Antialiasing Quality". It works fine for Plot3D
Plot3D[Sin[x + y^2], {x, -3, 3}, {y, -2, 2}]

but in a ListDensityPlot3D the axes and the box show step-artifacts
data = Table[Sin[x] Cos[y] Sin[z],
{z, -5, 5, 0.1}, {y, -5, 5, 0.1}, {x, -5, 5, 0.1}];
ListDensityPlot3D[data]

The same happens for Raster3D objects in general or when I combine Raster3D with a smoothed 3d plot.
Graphics3D[{Raster3D[RandomReal[1, {5, 5, 5}]]}]

ListDensityPlot3D. – halirutan Oct 27 '15 at 15:12