For me I have to say it was the mix of the comp scene and the amazing maps you would see at events. As a mapmaker I know that maps are what kept the community coming back to the game at least for the casual side.
When it comes to the comp side, I think I enjoyed those years the most since that's when some of the largest turnouts for leagues ever happened. (at least for NA) On top of that, how skilled players were was pretty even across the board. You could also train a regiment to improve and get better. I am not gonna diss on 2017-present or on newgens or on current wannabe top tier shits, but it seems that as people have aged out/left the skilled players seem to all bunch up. Perhaps that's what made the "Old Gen" era great. You didn't have just one or two really great regiments you had several. You had skilled, smart, and approachable leaders who didn't just care about their regiment but they cared about the community. I gotta give a hand to those regiment leaders/community memebers (whether I liked you or not) who actually helped the community grow and made the game fun. The rivalry, the fellowship, and the friends that I personally made in that time period really made it...."The good old days"