As @Lars said, you need to use separate path elements. Also, you could use a line generator so you don't have to create the path strings by hand.
var data = [
{p: [{x: 100, y: 100}, {x: 200, y: 100}], w: 2, c: 'red'},
{p: [{x: 100, y: 100}, {x: 150, y: 200}], w: 3, c: 'blue'},
{p: [{x: 150, y: 200}, {x: 200, y: 100}], w: 1, c: 'green'}
];
// Line generator
var line = d3.svg.line()
.x(function(d) { return d.x; })
.y(function(d) { return d.y; });
svg.selectAll('path')
.data(data)
.enter().append('path')
.attr('d', function(d) { return line(d.p); })
.attr('stroke-width', function(d) { return d.w; })
.attr('stroke', function(d) { return d.c; });
I wrote a small fiddle here: http://jsfiddle.net/pnavarrc/9Qqy8/
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