Yeah, it's rather similar to Dragon Age: Origins, with a central mission flow which opens up in the middle, before funnelling through to the end:

Eden Prime, then Citadel. After that, the game opens up to three main missions taken in any order: Rescue teammate, Feros and Noveria. After completing two of those, Virmire opens up, which you can complete at anytime. After that, you're on a straight path to the end.

Hilariously, this results in the situation where you can rescue the team-mate last, long after the other three primary missions.

Probably the best order is Teammate, Feros, Noveria and finally Virmire.