I had a these thoughts after hearing the news:
1) If you stopped the average person on the street, there's probably only a handful of movie critics that person would know by name. Ebert was one of them. That's significant.
2) Even when I disagreed with Ebert, I admired his approach. There's even an example or two of him changing his mind, or having his mind changed by a sound argument, which seems rare among big-name critics. Also, he was a lifelong newspaperman who managed to write intelligently about movies while keeping his prose accessible. When I stop and think about it, these are the things I actually liked most about him, not so much that I tended to agree with his assessments.
3) As has been mentioned, cancer is a motherfucker. It also whacked Siskel.
4) He seemed to see criticism as a conversation about movies, not a war of contesting aesthetic values. On TV, his tone of voice often carried the implication that "this is my opinion." Other prominent critics, from Kael to Armond White, reliably adopted a tone that suggested their opinion was gospel, the objective truth that you'd be silly to disagree with. It's a tone designed to end the conversation rather than start it. I didn't see him adopt that tone as much as others. (Although he did do it to Siskel a lot back in the day. It was their rapport.)
5) His memoir wasn't bad, but this e-book, about Siskel & Ebert's behind-the-scenes shenanigans, is goddamned fascinating.