What a great, thought-provoking question....
The top four, in my opinion:
1. I think the most important development was the creation of this website. This is the venue around which the FIRST community gathers. The evolution and growth of FIRST would have been much less efficient without the CD forums. Sure, others could have done it, others HAVE done it, but this is the original and best.
2. Alliances. Above posts already covered it well.
3. Mr. Francois Castaing's impact on Midwest (okay, Michigan) teams. It was he who persuasively twisted the arms of countless auto industry people to get us to where there are 3 Michigan regionals, many teams, and healthy support.
4. The "Indiana Mafia." Beatty/Hammond keeps winning regionals/nationals and their spawn go on to create great teams. Technokats get famouser and famouser due to hard work, sharing, Andy Baker, and others. More teams with great leaders and hard-working members build a community one team at a time.
coming soon 5. Explosion of Canadian teams. The descendants of 188 go on to do for Canada what Castaing and La Cosa Hoosier did for their regions. Thanks in advance, Mr. Breadner et al
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I'm sure there is a California story that parallels 3,4,5, but I don't know it....
Ken