OK, maybe the "tall" is a bit of a stretch, but the rest is a perfect description of Jamie.
I'd actually go so far as to say "dark" is a bit of a stretch - when I think "dark" I think Matt Bomer. Jamie has that sandy/ginger thing going. ;) But ultimately the physical descriptions aren't the point; the gist of his physical gifts + his personality type are what will make a great fit for Jamie. I can very much seem him pulling off the dichotomy of the type-A personality who isn't always the best co-worker, yet who is uncommonly good at reaching out to patients.
The physical descriptions, as you noted, are of even less concern with other characters - Jen Finnigan does not match the description of Tina, Alfred Molina is not as old as Harding is described, etc. etc. I agree that it's a bit disappointing that they felt they couldn't cast to type for Michelle, though I'm sure Emily Swallow will do a great job. One other way of looking at it is that certain archetypes play differently (ie emphasized) on screen than they do on the page, and on the show it might actually end up seeming unnecessarily awkward/mean if for some reason the one less-attractive female character also happened to be the one who struggles at the job, amidst all these other rock-star doctors.
Re: Original creations for the show: Are you thinking of Bill Irwin's character in particular? Was waiting for some clue about him, nothing yet.
As of the first eight pages, less so far about the character of Sydney; hoping for more of a sense of her really soon, as I've already taken a shine to Sarayu Rao because her tweets are hilarious. ;)
One thing I want to speculate on a little re: the Ty/Tina relationship; at this point in the book we understand it's there but we have very little detail to go on, I expect we'll get a bit more as the book progresses. I'd place a bet, though, right now that we'll get some kind of concrete resolution to it before the book ends - either she will go back to her husband, or she'll leave Mark but maybe it doesn't work out with Ty either. (I get the feeling like the realism of the book kind of makes the idea of a romantic Ty/Tina ending a long shot.) The reason I bring this up is that it will be interesting to me to see how the show plans on handling the conundrum of an extra-marital affair long-term. It's easy to keep a workplace romance between singles going indefinitely (See: Any number of couples on ER over the years). But when one of the people is cheating on a spouse, the longer you draw that out the more you run the chance of that character seeming less and less sympathetic, which IMO would be a serious concern considering that Tina is essentially the female lead. (There are plenty of ways to work this - break them up at the end of the season, give Ty another girlfriend, etc, none of which would prevent them from hooking up again later, esp. if the Ridgeway marriage does eventually tank.)
I'm sure I'll jump in with more later. Lunch break is over though. ;)