1) I loved that appearance wasn't limited by archetype, role, etc. One day, I decided - just fiddling around - to make the least-tank-looking tank that I could think of. I ended up with a little blonde girl in a pink-and-white dress. Then, just for fun, I wondered what her codename would be. When "The Pink Panzer" occurred to me, and I found that it was available on that server, I couldn't delete her.
2) RADIO MISSIONS! I loved radio missions! Not only did it provide a convenient source of quick missions for short-time play, but it filled in very nicely when a toon was "between" contacts (had no contact who would provide missions).
3) I do remember ONE mission that required teaming. It was given by a Talos Island contact (or the mission door was in Talos), and two widely-separated bombs had to be disarmed simultaneously. Once they made it possible to auto-complete missions, THEN teaming wasn't required, which suited my soloing soul just fine.
4) The fact that any archetype could solo. I got a storm/electricity defender to 50 while soloing the vast majority of the way. (I couldn't resist teaming her during ToT, though. I loved using Freezing Rain for that; it just never stopped being entertaining.)
Man, I'm homesick right now.