BioWare knew that many Mass Effect fans would not open its multiplayer mode with open arms. The online co-op mode ties into the main campaign, and offers players an alternate way of unlocking the best ending to the trilogy. In spite of all the hoopla surrounding the mode, it is essentially optional.
Even the game's achievements reflect the optional nature of the online component. If you refuse to play co-op (or you simply don't have an Xbox Live Gold account), you'll still be able to collect every achievement.
For example, "Tour of Duty" has you playing all the multiplayer maps, or playing every "N7 mission" in single player. "Explorer" has you playing three multiplayer matches, or finishing five N7 missions. "Peak Condition" will unlock when you reach level 20 in multiplayer, or level 60 in single-player. Finally, "Unwavering" can be unlocked when finishing all the multiplayer maps on Gold, or all single-player missions on Insanity.
Having online-only achievements would have been an easy way of forcing Mass Effect players into trying the multiplayer mode. BioWare's flexible approach to achievements is definitely much appreciated. See all the achievements at X360A.
Mass Effect 3 will be available on PC, PS3, and Xbox 360 on March 6th. You can try out a demo (which includes a taste of the online) soon.