I totally agree that losing Tywin, Oberyn, Sandor and Bronn (at least for the time being with the latter two) is going to have a negative effect on the show. Especially on the heals of losing Robb and Catelyn (AND JOFFREY!) Tywin and Sandor are two of the best anti-heroes/villains in the show and Oberyn and Bronn are definitely charismatic fan favourites. I know I was disappointed to lose Oberyn so quickly in the books.
I think the show will be hard pressed to get fans invested in the Greyjoy storyline at this point, having not introduced some of the major players in that storyline yet. When I read Crows and Dragons I enjoyed the history and subplot aspect of the Greyjoys plotline but to be honest I'm not too concerned with any of their fates, other than Asha. I assume their doom is inevitable so I'm not worried about them dying, only how they effect the overall plot (ie. dragons) before their end.
I think the new fan favourite characters will come from Dorne though. I loved finally seeing Dorne in the later books and I think Arianne, Doran, Quentyn, Areo and the Sand Snakes will all become favourites.
Plus the show will benefit from combining books 4 and 5 so we don't go a full season without seeing Tyrion, Daenerys, Jon etc.