Spoilers if you have not read the books
Several semi-important characters are conspicuously absent from the show making some scenes rather awkward later on in the series (as dialogue is swapped to other characters.) Obviously this is difficult for the show since the books have far too many characters but there are 3 which I noticed who are set up in the first book and play significant roles throughout the next 4 books.
<PERSON>, <PERSON>'s uncle is probably <PERSON>'s most important advisor, and his absence means that <PERSON> seems to be surrounded by a very undistinguished bunch, Great <PERSON> is a bit of an oaf, <PERSON> is a sleaze etc
<PERSON> asks for his squire, to which <PERSON> replies he doesn't have one. This is a bit of a wink at those who have read the books because <PERSON> is <PERSON>'s squire and plays a minor but continuing role throughout the next 3 books, and when last we saw him was left in a cliff hanger alongside <PERSON>. I wonder if <PERSON> knows he is unimportant later on already or if the show writers figure they could just write him in later if required.
<PERSON> is probably the most important character to be missing from a thematic point of view because he keeps the North Men from being the 'good guys,' and shows the readers that in war every side has their heroes and villains (or at least villains.) Not only is <PERSON> gone, but so too are the 10,000 men he is supposed to be commanding. 190
Rather than winning an indecisive victory against half of <PERSON>'s force left under <PERSON>'s command, and then having to withdraw to gather his strength at Harrenhal, <PERSON> crushes 2000 men left to die by <PERSON>. This really doesn't make much sense, since first of all <PERSON> has fewer men than the lannisters to begin with, and why send 2000 to whole sale slaughter anyway. More to the point it would be pretty reckless of <PERSON> to leave the Twins and the Neck completely open to attack by <PERSON> (since he wouldn't know of <PERSON>/Renlys plans.)
Geography is not really focused on in the show though so some license is reasonable. However leaving <PERSON> will affect the Brave Companions, <PERSON>, <PERSON>, <PERSON>, and without <PERSON> introduced the events in the north involving his bastard son will also be a bit out of left field. 190