I'd compare Mass Effect to the Matrix in terms of being part of a trilogy.... They closed the plot and settle everything, but it's still open for a sequel. Like in the first Matrix. They could have ended it all there, but they decided to make it a trilogy, (and ruined it I might add). Mass Effect is the same way, where they could probably just end the series there, but they won't because there is one issue left, and people love it so much and want to keep playing to find out what happens.
It is NOT a cliffhanger like Crysis... Crysis is also set to be a trilogy, but they totally leave you hanging at the end. Mass Effect does not leave you feeling like that at all.
Not only that, but Mass Effect has replay value. Assassin's Creed is so damned repetitive, you are sick of it by the time you reach the end... And Crysis is so linear, you really don't feel like replaying because there won't be any surprises. Mass Effect actually plays out like a different game if you choose to make even the smallest decisions. I played it a second time immediately after I beat it the first time, just because I wanted to see what I missed.... Past 2 times, though, you may find yourself looking for something new... I might re-install Crysis



