I've been playing for about ~16 years, GMing for 12. Baldur's Gate got me into tabletop and I never looked back. My favourite systems (in no particular order) are DnD 3.5, Dark Heresy & Rogue Trader, World of Darkness games, Pathfinder, Dresden Files FATE, Wheel of Time d20, Star Wars, Witchcraft system rpgs, Shadowrun... Okay so maybe I game a lot.
I think the role of a GM is to empower players to make meaningful (and sometimes difficult) choices that affect the world they game in in visible ways, in settings and contexts they find stimulating and satisying, and fulfilling in intellectual, artistic, and emotional ways. As such I tend to include sandbox regions (with the plot moving in the background on a schedule, so dawdling with sidequests is also a choice), difficult moral choices, tricky combats that require cleverness and lateral thinking, and high amounts of player agency. I also most enjoy games that include these elements.
I think the second role of a GM is to create a nurturing environment for newbies to enter and accomodate themselves to the hobby, whatever their system or style of choice. Too many gatekeepers alienate too many people, and I try to encourage newbies as much as I can. :)