I am generally a Ridley Scott aficionado and yet I was disappointed too, although I suspect the film's shortcomings have a lot to do with its troubled genesis. As previous posters have pointed out, it was originally entitled Nottingham and was meant to be a revisionist take on the Robin Hood mythology, with the Sheriff as the good guy and Robin as some sort of medieval proto-terrorist. Scott himself apparently saw the Sheriff as a conflicted hero torn between his duty and his personal feelings. At some point in pre-production though, the studio (or possibly Russell Crowe wearing his producer's cap) got cold feet and decided to play it safe by reverting to a much more commercial formula, complete with one-dimensional villains and a politically correct, democracy-loving hero liable to appeal to audiences on the American market.
Although several writers were successively involved, there were still major problems with the screenplay and numerous rewrites ensued, which delayed shooting by several months. Tom Stoppard was even called in to rework the script while Scott was filming, a sure indicator that the production did not quite know where it was headed.
The film's lack of fire and conviction may well have a lot to do with this erratic pre-production history. Although he handles the visuals in his usual assured manner, Scott does not appear to be terribly convinced by the script he has to work with. The net result is that everyone involved is playing it by the numbers, with the exception of Mark Strong who does a dashing villain with just the right touch of complexity and ambiguity. He was certainly the best and most intersting thing in the film for me.
As for the 'history' of the film and its Saving Squire Ryan climax, it's best not taken seriously, although it's not entirely unreasonable on the part of the audience to expect the filmmakers not to rewrite and reinvent said history altogether. But I very much doubt there would be a point in explaining Ridley Scott and company that it was John Lackland who tried to invade France after Richard's death rather than the other way around.