TopoJSON is a variant of GeoJSON that supports shape simplification, map coloring, cartograms and other topology-related features.
https://github.com/mbostock/topojson
For small geographic features like tiny islands, try the new d3.geo.tile plugin with a high-resolution tileset.
http://bl.ocks.org/4162068 https://github.com/d3/d3-plugins/tree/master/geo/tile
TileMill can create tilesets from shapefiles and other formats. It can export GeoJSON for D3 too.
More examples:
http://bl.ocks.org/mbostock http://bl.ocks.org/jasondavies http://bl.ocks.org/tmcw