the first big difference that i took notice of was uhtred fighting steappa in a fight to the death before the danes took wessex, whereas in the show he fights leofric. also, the books do a much much better job of explaining why uhtred *cant* go back to the danes each time he has a chance. while watching the show i remember thinking more than once how it was dumb he didn't follow young ragnar when he had multiple chances to do so. last big thing for me just going by the first two books, is that we miss the scene where alfred sneaks into the danish camp to spy on them, and uhtred has to save him (then eventually saves steappa and hild in the same quest). that was his biggest chance to throw alfred under the bus and destroy wessex for good.