reaching 100% Galactic readiness? I planned on just getting the cheevo, but now I'm hooked. Also, what's the highest level you can rank? I seem to be stuck at 20.
the max level for any character in MP is 20. You can promote the class and start from scratch, but will never level above lvl 20. Once reaching lvl 20 you stop gaining exp. --- Currently playing: ME3, GoW2, Twilight Princess
Some people only play until they've reached 100% when they have an active singleplayer game going, others play regardless either because they enjoy it or because the singleplayer was just that bad (as someone has mentioned). Though I do know one guy who seems to think he has to have 100% even though he still has more then enough assets when sitting at 50% but what ever.
As for how the whole ranking system works in the game. Each class can only level up to 20, from there you stop getting experience. Each level will increase your N7 rating (the red bar) and when you promote a class (this will reset every character's level in that group) to start over again you gain an additional 10 points to your rating. They will also add to your War Assets in the singleplayer (not sure if it's a set amount per promotion or if it varies) making it easier to get the requirement for the "best" ending which i think is like 3100 or 3400 EMS, something like that, it's pretty easy to get since they released the Extended Cut (originally it was almost double that). I guess I'll also point out that the numbers in the yellow bar is the challenge points a player has and the blue one is the number of times they've promoted that class. Banners (the pictures that sometimes look ugly as hell and make it hard to see what the player is playing as without checking their gear) are gained by competing challenges. You can add one to yourself in the Challenges section in the multiplayer menu once you've met the requirements. However weekend challenges don't give you banners unless stated in the operation description.