I just finished my third play-through of the entire Mass Effect trilogy. That's a lot of time to spend with a single universe, even with one as big as Mass Effect's is - after all, there's only so much you can pack into a largely-linear experience, especially one with a very definitive beginning and a very definitive end (shitstorm about the series' peculiarly homogenized triple-ending notwithstanding).
For someone not particularly fond of big 'sit down, let me tell you a story'-type games, there's a few things that brought me back to Mass Effect more than once:
I could skip (almost) everything
When the primary means of imparting information is vocal or otherwise time-consuming methods, you tread a very dangerous line - drag it out too much, and the player gets bored; rush through it, and they may miss critical bits. The series did a fairly good job at avoiding both extremes in every game. Being aware of the general lines (and how I wanted to play the game) meant I could skip through a large portion of the chit-chat, pausing at critical choice moments - either deliberately or the game forcing it - and then get back to the action.
The unfortunate side, though, is that the critical cutscenes and other 'forced-must-watch' are some of the more boring ones. I like to max out the difficulty, so I died a lot during intense battles; and seeing the same boss intro or hearing the same...