*turns 'Mr. Roboto' song on*! Sorry, couldn't resist.
Such an open question, but that is a cool question. I like a lot of the points made here especially from dr. syntax. My 2 cents I guess... its small, 4 valence, it may covalently bind to itself (building blox!).. electronegatively pretty neutral, it can go oxidation +2, -2, etc.
Silicon has 4 valence e's also, but its so much bigger... I think that's the main difference it'd have a tough time fullfilling same role as C, it couldn't bind to N, O, H, etc. like carbon can, if you think how friggin huge some protein complexes are there's prolly just no room >.<