One missing consideration here seems to be the overall group dynamics.  Hanging out with a group of friends that have all taken molly for example is a totally different experience than hanging out with a mixed group where some are rolling and some not.  

I've noticed the same occurs with alcohol to a slightly lesser extent.  Ability to mirror the energy, jokes, conversations, etc. when not in the same state is often reduced (sometimes significantly).

For large parties, this effect is minimal to non-existent, but with smaller groups, I've found it can change the overall group dynamics.  Hard to find the right word, but it can lead to less comradery and a less cohesive experience for every member of the group.

Agreed, this is probably the best lesson of all.  If the buttons exist, they can be hacked or the decision makers can be socially engineered.

270 people might have direct access, but the entire world has indirect access.

How about the invention of the Internet?

No celebration since it doesn't really have a clearly defined date, but I'd argue it is relatively recent and may have impacted society more than any of the events you listed. It's amazing that the majority of humans on earth now have access to such a wealth of information.