What was the first character you played in an RPG other than D&D? How was playing it different from playing a D&D character?
This is a bit of a problematic question for me to answer. The reason being that, as mentioned previously, it took me some time to get a regular group going despite being extremely interested in RPGs for a period of two-plus years, and that one of the ways I kept involved in the hobby during that dry spell was by making characters for the fun of it.
I made my fair share of D&D characters, but I also made characters for a wide variety of other systems: GURPS, Rifts, Call of Cthulhu, Ninjas & Superspies... I still have some of those test characters in a folder somewhere, sitting there unplayed even nearly 25 years later, poor chaps.
One thing that all those test characters taught me, however, was how much different systems could vary, even if they were broadly similar at first glance. Palladium games used game mechanics and a class-and-level system very similar to D&D, but the characters I made for those games felt vastly different than the ones I rolled up from the Players Handbook. To this day, simply making a test character for a new system remains one of the best methods, in my opinion, to get a quick read on the game's mechanical and narrative priorities, and to familiarize oneself with the core mechanics of the system at the same time.