I'm glad to see my vote is part of the majority: The Genesis Block was the birth of Bitcoin.
Although nobody voted for the registration date of bitcoin.org, I'll add an argument against it: a domain registration can simply be ended. If bitcoin.org disappears (or starts selling car parts), that doesn't mean Bitcoin ends. Satoshi's whitepaper will never disappear, and neither will the Genesis Block.
Can we be confident in saying that the mining of the genesis block was a considered event?
In retrospect: yes! It makes perfect sense that not all historical events had a large impact the moment they occurred. Think about great inventions like steam engines, steel, electricity or transistors: it took a while before it become common.
Now let's see if Bitcoin can become mainstream in the next 10-20 years!