Yes. The core mechanic - 1d20 plus modifiers compared against a target DC - is simple and flexible enough to apply to almost any situation. Everything else can be added or taken away to make it as simple or as complex as desired. You can cut feats, re-define skills, chop and change the magic system, mess with races and classes, add and remove things a PC can do, and it'll still be d20.
I've played a few other systems (GURPS, Lejendary Journeys, Tunnels & Trolls, Maelstrom, Fighting Fantasy, Dragon Warriors) and I own and have read but have not played a few others (ARIA, Lost Souls, various White Wolf games, In Nomine) and find that in terms of rules mechanics, I like the d20 system better than any of them. For me, it strikes a good balance between realism (factors that influence outcomes are roughly mapped to what you expect in the real world) and convenience (simplifications and conventions to make the game run more smoothly), flexibility (ability to model different types of characters, objects, challenges and scenarios) and structure (pre-existing "packages" that can be assembled quickly and easily to make characters, objects, challenges and scenarios).
I also have to admit that when I was younger, and more innocent and idealistic, I tried to create an RPG game system myself. It was a very complex system (I had somehow acquired the idea that complex and realistic == good), with 10 character attributes ranging from 1 to 100 (the player would roll 11d10-10 for each attribute, and rolled another 10d10 which he could add to any of the attributes), weapons dealt damage equal to a number of d10s (a dagger might deal 2d10 damage, for example) and armor absorbed damage equal to a number of d10s (leather armor might absorb 2d10 of damage, for example). To resolve an attack, you would roll d%, add your attack power and your weapon damage, and your opponent would roll d%, add his defence power and the damage absorbed by his armor, and if your roll beat his, you did that much damage.
Somehow, the idea that any system that required the rolling of 120 dice during character creation, and required the adding up of several double-digit numbers to resolve a single attack might be more trouble than it was worth never crossed my mind...
