Some thoughts.
The thing that strikes me the most is how much of a giant he was in the field - if there was any face you could attach to comics, it had silver hair, sunglasses, and a razor moustache. He was Marvel since before most of us were born, and got to see his creations turn into worldwide pop culture. When I was little, I read my dad's old comics from the 60s and 70s, Iron Man and Captain America both. That was my intro to the Marvel universe.
It's really hard to put a finger on just how much of a combination of creative genius and media leader he was, I suppose; the only real comparison I can think of to what we've lost is when Walt Disney died. A name that meant more than a brand, a name that meant a genre and an entire media. And over the last ten years, a media that's become a planetwide icon.
I think that's why it hurts so much - because everywhere you looked, at conventions, in the comics, in the movies, in the TV shows, there he was...until today.