BioShock did have levels with start points, end points and an order to do things in, but the levels were actually pretty wide open. They allowed you to go almost anywhere at any time and you needed to do lots of backtracking and exploring to complete them. You could also approach many encounters from a different direction, run from them and/or lure enemies into a trap.
In contrast, Mass Effect did let you choose which planets to go to and in which order, but the actual missions in them were extremely linear. They consisted of going from room to room in a specific order and getting ambushed each time by enemies coming from the other side of the room, with no option for running, approaching from another direction or doing the ambushing/attacking first, instead.
I find the latter boring and uninteresting. What's the point of having the choice of which missions to do and in what order if they're all the same? Once I realized which missions in Mass Effect were side ones, I started skipping them. I'd rather have the same order of objectives as everyone else, but the freedom to tackle them in the way that I prefer. Of course, I really prefer having freedom of mission selection and freedom of execution (such as in some true RPGs, like Bethesda's), but every game doesn't need to be open world like that. A game does need to have good gameplay, though, and I feel that BioShock has that and Mass Effect doesn't.
