A common problem with line-simplification algorithms when applied to polygons is that they can introduce self-intersections, which generally cause havoc with geometry algorithms.
It's quite possible that your simplified polygons contain some self-intersections, e.g. a segment that goes back on itself. This might cause problems for D3, e.g. when sorting intersections along the clip region edge (although in future releases I hope to support self-intersecting polygons, too).
A better algorithm to use might be Visvalingam–Whyatt, e.g. as used by TopoJSON, as it simplifies based on area. However, it can also produce self-intersecting polygons, although perhaps less often than Douglas–Peucker.
For interactive globes I’d recommend world-110m.json
from Mike Bostock’s world-atlas.