I think the issue with Bloom is more of a casting problem, rather than him. Yes, there are others who would have been better suited for the role, but I don't think he was "awful". His character was not well written or interesting, I agree.
But then again, in the grand scheme of things, the film with it's simple setup, clearly shows us that it's about the political situations, the intrigue, the religions, the wars, the fighting. There is little development for him.
In Gladiator and Braveheart, you had a tale of revenge, about a man who goes through a journey, and all these patterns until the story reaches it's conclusion. In, KoH we don't spend much time with the main character. He is ignorant about many ways, and that's how the situation is being explained to us. By the protagonist being in an unfamiliar place.
I think it was trying to go for something else. I was not bothered by Bloom at all. I was way to focused on the charisma of Edward Norton, Jeremy Irons, Brendan Gleeson, Eva Green and Liam Nesson. I also loved the actor who played Saladin. Very, very nice characters and performances.
I am not excusing Orlando Bloom or trying to say that it couldn't have been MUCH better, but this film was clearly more driven about historical events rather than character development, were Gladiator and Braveheart(to my personal knowledge) were much more character drama based films with loose inaccurate historical plots. Not that there is anything wrong with that. Just apples and oranges.
KoH

C Reminds me of the classic fairytales about the hero who goes out on adventure and sees many things. Usually the hero, is not very interesting. He is more like the indifferent hero that puts the audience in the boots of him, and let the audience see the world through his eyes. It's just a character, who is an excuse for the camera to move around in all these interesting places and meeting these interesting characters.
That's just how I see it though. On a final note, I actually think that Orlando Bloom could really be something one day. If he gets the right part, and is being directed probably.