While Mass Effect was an RPG masquerading as a shooter and Mass Effect 2 was a shooter masquerading as an RPG, Mass Effect 3 tries to split the difference between the two. The combat system from ME2 has been largely unchanged, though Shepard now has the ability to make evasive combat rolls, and her melee capabilities have been greatly enhanced with the new OmniBlade.
Weapon modification is back, as are an almost unnecessary amount of options to pick from, especially when combined with the 8-10 different versions of each weapon type (each with 10 levels of upgrade). About a third of the way through the game, I just stopped trying to figure out which weapons would be an upgrade and stuck with what felt comfortable. You can also customize Shepard's armor with a variety of stat-boosting pieces, but none of them ever felt particularly substantial to me, so I ended up aiming for a balance of small upgrades vs. what looked the coolest.
While the storyline is engaging throughout, a number of rather inane sidequests have also been added and tied into both the fan-favorite planet scanning minigame and the new Galaxy at War metagame - more on that later. There are a good number of sidequests that play out well, but the vast majority of the ones you'll come across are randomly discovered when you walk by people in the Citadel and overhear their conversation. These usually involve finding some macguffin in the planet scanning mode and returning the item to the character, who is understandably surprised that you happened to find that item during your travels. I wanted to skip these, but they are tied in to your war rating and (again) the Galaxy at War.
Planet scanning has been simultaneously streamlined and made more annoying. While searching for various sidequest items, the Normandy can send out a scanning ping to detect whether or not there's something to scan for. Send out too many scans and you'll be greeted with a fleet of Reapers chasing you down like a lame game of Pac-Man. If you get caught, it's an instant game over. Luckily, the game quicksaves for you each time you enter a new solar system, so you won't lose anything but load time.
The Galaxy at War metagame measures two things: Shepard's effective military strength and the preparedness of the galactic militaries to fight off the Reapers. Your military strength is increased by gathering war assets through questing, and is affected by the aforementioned preparedness stat. The galactic preparedness level is set to 50% by default and can only be increased by playing co-op or ancillary products such as Mass Effect Infiltrator (iOS). Increasing preparedness is the easiest way to boost your effective military strength (which ties into events leading up to the ending), and thus becomes the target of more gamer ire, with accusations of forcing people into playing the multiplayer mode in order to get the "best" ending.