The Bioware blog recently posted about this information, which I'll summarize here.
Mechanics
Earning reputation in Mass Effect 3 is a bit different from the first two games. Rather than earning only Paragon and Renegade points, you can also earn Neutral reputation, so there are 3 types of rep in ME3.
This means you can now mix being Paragon/Renegade without worrying about getting locked out of the more difficult Charm/Intimidate options. When you earn Paragon/Renegade rep, it'll show up as Blue/Red, whereas if you earn Neutral rep, it'll increase your reputation bar's size as a whole while retaining the same blue/red ratio.
Click through for the larger version.
Your total reputation (blue/red/neutral) will determine whether or not you'll be able to pass a Charm/Intimidate block. The lines on the bar are "checkpoints" which serve as gates for more difficult dialogue options. I marked them with the thin lines of red, in case it was difficult to see (this is not my Shep, this is the photo from the blog post).
As per usual, you get rep from completing missions and making dialogue choices. For missions with no Paragon/Renegade choice but still have a large impact, you will get Neutral rep.
Story Impact
In contrast to older Bioware games, you can never actually be the villain in ME3, though this should be rather clear from how they handled the first 2 games. No matter what, you will be fighting to save the galaxy. The manner in which you do it will be what differentiates a Paragon from a Renegade. Also, bullying people into giving you more money or discounts won't be the crux of what makes a Renegade decision in this game. Who needs credits if there's nowhere to spend them because civilized life has been destroyed?
Reputation gain will be based on when you have to make a decision in the game. Here are a few examples of what kind of rep you would get, taken from the blog:
- Confronted on the Citadel by a desperate refugee with a gun, you give her some credits and help her find a place to sleep. (Paragon)
- As a human colony falls to Reaper forces, you order down an orbital strike, brutally killing thousands of colonists to prevent the Reapers from turning them into husks. (Renegade)
- You land at a turian fuel depot taken by Reaper forces and clear it out, enabling allied forces to keep fighting. (General Reputation)
TL;DR
BioWare has finally made a single-player game that lets you make morally grey choices without nerfing yourself, gameplay-wise. Rejoice!
Saving the base gives you 110 points towards your Galaxy at War score compared to 100 for destroying it.
There is a rumour that the Illusive Man will be friendlier towards you in ME3 if you went his way in ME2 (saving the base) but that's unsubstantiated.
Since he's (confirmed) trying to kill you in ME3, I don't know how much weight this carries.
The endings below 1750 or above 2800 effective military strength are the same, regardless of what you chose to do in ME2.
The endings, organized by rating required are different depending on what path you chose:
Low EMS Option
Saved the base:
1750-2049 EMS Choose to Save the Reapers and the Earth will be saved.
Destroyed the base:
1750-1899 EMS Choose to become a Reaper and the Earth will be destroyed.
Mid range EMS Option
Saved the base:
2050-2349 EMS Choose to become a Reaper and the Earth will be saved.
Destroyed the base:
1900-2349 EMS Choose to destroy the Repears and the Earth will be devastated
Saved the base only option
2350-2649 EMS Choose to destroy the Reapers and Earth is destroyed.
High EMS Option
Saved the Base:
2650-2799 EMS Choose to destroy the Repears and the Earth will be devastated
Destroyed the Base:
2350 - 2799 EMS Choose to become a Reaper and the Earth will be saved.
Best Answer
There's two phases to scanning for War Assets:
On the system map, you can scan a region of the system by sending out something that could be compared to a "sonar ping," and once you've located an item on a planet or moon, you can select that planet or moon and begin the planetary scan.
In the first phase, you press a button to scan a region around the Normandy. Any items of interest will be highlighted with a red ring. You can then go and investigate these sites. You can only scan a portion of the system at a time, and scanning too many times in the same system (if is is under Reaper control) will lead to the Reapers chasing you out of it, lest you game over.
Some items you will just find in the system map - items like extra fuel from the remains of a wrecked ship, for example. Others will require you to scan the planet before you can locate the item.
If you must scan a planet or moon, you enter the second phase. The second phase is very similar to the way mineral/anomaly scanning worked in Mass Effect 2. You'll be shown a 3-D model of the planet, and you'll have a little radar icon that shows you which direction the item in question is. You'll have to move over the surface of the planet, scanning periodically to keep moving in the correct direction. Once you've located the item (noted with a white dot) you can launch a probe. The probe will recover the item, and then you can leave orbit.
The scanning is quite a bit more straightforward and fast compared to ME2. In that game, you might spend 10 minutes extracting resources from a planet, and use up probes that had to be replenished. In this game, there's usually far fewer items to be found on a planet, and you have infinite probes.