The side quests have timers? They expire?
So, I don't want spoilers, but what is the premise of the game? I don't like playing and learning after a play through how it works only due to trial and error and because they didn't explain it to me well enough.
ME2: get squad, get loyalty, save IFF for late in the game, and don't enter Omega relay until the end when you are ready. Is there a similar trend for this game? How does the galactic readiness thing work? Do missions in those areas and balance them out to get them all up to 100%?
Side quests, scanning and main quests will add fleets, platoons, powerful allies to your military strength. They each have a value.
A reporter might have a value of 5. A fleet might have a value of 250. A Shadow Broker Wet Squad might have a value of 100. Basically, as you collect each of these assets for your war, they add up to a total.
That total is then multiplied by the galaxy's "readiness rating", which is a percentage of how much of the military you've collected is ready to fight the Reapers.
Your readiness rating defaults to 50%. Military Strength x readiness = Effective Military Strength, or EMS.
EMS is the value you have to pay attention to when getting close to the ending.
You can ONLY raise your readiness by playing multiplayer. You don't need to raise your readiness for the good ending, but it makes the SP easier.
Think of it this way. Let's say the best ending requires 3000 EMS. You can either do two things: Play only SP, or play both SP and MP. If you play SP only, you would NEED 6000 Military Strength to get 3000, since your readiness would be sitting at 50%.
Now, if you play multiplayer, your readiness increases by about 4% every time you finish a bronze (easy) difficulty battle. Readiness also decays at a rate of 1% per day. Which is fine, because playin a single bronze level per day means that you're gaining readiness faster than you're losing it.
So if you play enough MP to get to 100% readiness, you suddenly only need 3000 total military strength to reach your EMS of 3000. So that means doing less side quests, scanning for less assets, etc etc.
Does that make sense?
On the topic of side quests, side quests don't have an actual timer, but can be failed if you don't do them BEFORE doing priority missions in the same system. Priority missions are story missions, and they will move the story forward, which means circumstances will change in the galaxy as you propel the story.
Basically, it's a good idea to do side quests you can do before taking up story missions. That said, you WILL get side quests that you cannot do until a star system opens up for you via the story. It's a little annoying.