In my opinion there are two main things to CSI:
It's a serial interconnect, which should simplify routing in MP boards and allow point-to-point CPU connections instead of having all inter-cpu transfers hit a FSB hub (exactly like HT did for AMD).
It's propietary, which should allow Intel to keep its iron grip on its chipset market and makes sure that Intel's chipsets are only used on Intel CPU-based systems (the main drawback to AMD is that its chipsets are not Intel which, in my opinion, are the best ones out there).