a) .com is where all users go by default. "Go check out ycombinator" means to you that it's a .com b) .com has a ton more value for search engines...once again

a) .com is where all users go by default. "Go check out ycombinator" means to you that it's a .com b) .com has a ton more value for search engines...once again the same idea...if you have a .com, then you are the real business...if you have the .co you are just a pale imitation c) .com has a ton more monetary value, creditcards.com might sell for 20 million, creditcards.net, might sell for 20K