Actually, any system is fine by me as long as they don't ruin it: dialogue options(Reject, accept, deny, attack, hand over item, keep item, etc.) changing positions thus confusing the players, very lengthy dialogue lines(keep it short and sweet, please!), etc.
Well that's a difficult one, some like more the dialogues of Diablo2, other ones admire the dialogues of Planescape Torment. But it's handy if you have a bit of a choice, but the dialogues were good in DD and BD, lengthy, but you could skip quite a lot till you need to reply. Then you choose your reply, and even if you didn't read it all, you could still just check your quest log to see what exactly needs to be done.