At this point, it shouldn't be a surprise that there are multiple endings for Mass Effect 3. What may surprise you is that to obtain the "best" ending in the game, you probably have to play multiplayer. The ending you get depends on your war assets and readiness level, which multiply to determine your effective military strength. This leaves a sour taste in my mouth for a "single player" experience.
You can get enough war assets through single player to obtain the ending, but it is very difficult. You basically have to complete every quest in the game. Essentially, the game forces you to play multiplayer co-op in order to complete your single player experience. This tactic seems a little underhanded. Maybe it's a way to drive the sales of unlockable multiplayer items. I don't know.
It is possible to get the best ending in single player without playing multiplayer, but it’s hard. All your war assets only count for 50% of their potential value. The biggest gains in War Assets come from a culmination of your decisions in the previous games and your decisions here. If you’ve helped a race before, and you help them in this game, you can get their full support and resolve their conflict... possibly even getting someone else on your side.
In other situations, a wrong call somewhere along the line or a Paragon or Renegade score too low to pick the right thing to say, can mean a character dies, a race is demoralized, and if they fight for you at all they’re worth much less to the war effort.
All this isn't to say that Bioware didn't make a great game. They did. A lot of the complaints over this readiness and ems issue seem to stem fro the fact that people just aren't happy with the game's ending in general. I just wish the culmination of your single-player efforts didn't depend on a multiplayer experience. I'd rather just see a focus on the story. It seems like another strike against EA/Bioware that has already come under fire for things like Origin restrictions on the PC, DRM issues, and the day-one-DLC fiasco.
