The Ultima games and me go way back. I was first introduced to Ultima at an old friend's house. His older brother had a Commodore 64 and a huge stockpile of games - Rings of Zilfin, Adventure Creation System, and several Ultimas to name just a few. One afternoon we did not have any D&D adventures prepped to play so he told us he "had something that we could play that was almost as good."
I am not sure which Ultima we sat down and played that afternoon - somewhat irrelevant because I have played them all by now - but it was not difficult to see that this game was different from the computer rpg's we had played previously.
There were several things that stuck out immediately about Ultima. The world was open and available for exploration instead of forcing your character down a strict path. There were many options for interaction in the game. In the earlier entries, your character could ask the citizens their name, job, and other information. In later entries of the series, you could basically have your character carry on a full conversation by using keywords that are highlighted. There was also a morality mechanic, recurring characters, and an overarching narrative to several of the entries. If you have never played any of the Ultima games, do yourself a favor and find one now.