I agree with Borek; not only was Librescu from a different generation, he didn't grow up in American culture, thus making the point kind of moot. He was certainly a hero, though.
On one hand, you can sit back and say 'why didn't anyone fight back?' Unfortunately, it's easy to sit back and say that from the comfort of your chair. You don't know how you're going to respond to that kind of emergency situation until you're in it. I wouldn't doubt the 1 in 5 actually fighting like in Vietnam. I think that much of that has to do with generational culture in the US, but that's a very difficult and complicated topic.
That being said, for those of us who have been in crisis-type situations where fight or flight was unavoidabe, it is a valid point to wonder why so many people were gunned down without much resistance. With the number of people that were in harm's way, I'm surprised there weren't more who did fight back. Personally, I'd have been throwing desks or books or whatever at the guy, but that's me and I have had experience in my life to know how I would react to a life-threatening situation (I'm not going to claim that I've had a school shooter come at me, as I doubt anyone on these forums has).
Interestingly, from what I've been able to find online, he only had 9rnd capacity in his magazines--which means he either had a lot of magazines or reloaded really quickly.
This whole situation is very sad. One thing that needs to be addressed is why he was adjucated mentally defective and still able to purchase a firearm legally. From what I've seen of this kid, he probably would have obtained one illegally, or made bombs or whatever if he hadn't been able to buy a gun, but to sell him one given his prior mental history run-ins with the law is pretty bad.
BTW, we're not the only country with shootings like this:http://news.scotsman.com/latest.cfm?id=687602007
That's from the UK--where guns have been illegal for a long time and the control laws are very strict.