"In what system was the first character you played in an RPG other than D&D? How was playing it different from playing a D&D character?"
Again it seems I'm going to have to dodge my inclination to talk about Pathfinder and the absolutely epic tale of Alfonse Mycroft, because of my still-held belief that Pathfinder is still basically D&D, but empirically better.
Therefore, I'm going to talk about my first game of Call of Cthulhu as a player, where I ran a shipping magnate named Simon Bailey with David Larkins of the RPG Corner as the Keeper of Arcane Lore.
The first deviation from playing a D&D character came with the setting, 1920's San Francisco. The second deviation was when I realized that I could use actual historical knowledge to create a backstory for the character; and thus Simon Bailey started with a 50% in rifle (which he never used), because he served in the French Foreign Legion during the Great War.
Playing an investigator, rather than an adventurer, opens up different avenues for character interaction, as you're focused more on negotiating relationships and solving mysteries with your wits, rather than your sword blade. Brute force is all-but passé in the 1920's, after all. I spent more time thinking about Simon's daily life as well, because as a Call of Cthulhu character you choose an Occupation for your toon, or PC, or 'guy', and therefore- unlike D&D- your occupation is not "adventurer" or "investigator" (usually). Chances are, you're investigating in your free time, and the rest of your days you're working as an antiquarian or professor or beat cop. It's a dichotomy that presents an interesting requirement for roleplaying that isn't commonly utilized in Call of Cthulhu games, mainly because most of them are short-form.
Finally, playing a Call of Cthulhu investigator incites various emotions to its player, as you're essentially running a doomed character. Madness increases throughout play, especially as you delve deeper and deeper into the Mythos, and your range of control over them becomes smaller and smaller as sanity rules take over your play more and more.
All in all, it's a great deviation from the D&D "norm" of roleplaying.