I dunno, I think the definition of "good speaker" really depends on the audience. I probably speak for many here as an introverted, deeply introspective outlier.
I have seen many great speakers in person (Tony Robbins, Zig Ziglar, Deepak Chopra, Steven Covey) and almost always come away underwhelmed. I struggle to understand why the audience gets so worked up with so little content transferred. I have trouble with comedy clubs because so many people howl at stuff I think is lame.
On the other hand, I find tech talks that would bore my friends to death incredibly interesting. I've seen pg speak several times and I really enjoyed his talks. I even like the "ums". They tell my subconscious to pay attention because I'm being treated to something real-time and genuine that has never happened before and may never happen again.
Oddly, my favorite tech speaker in the past few years was Reid Hoffman. He sure doesn't look like a professional speaker; he paced back and forth and mumbled with his head down. But I was afraid that if I dropped my pencil, I might miss something that could change my life. Now that's what I call a good speaker.