Mass Effect 3 is a third person shooter and RPG. There is a story mode which wraps up the main story from the first two games as well as a new multiplayer co-op mode.
Combat is mostly cover based, firing or using powers will bring you out of cover and leave you vulnerable. Certain enemy explosives will bypass or destroy cover as well.
There are a variety of weapons which fit into five different categories; sniper rife, assault rifle, shotgun, smg, or heavy pistol. Each weapon type has its own strengths and weaknesses which can be augmented with attachments. Attachments (like the weapons themselves) can be bought or found, acquiring them more than once upgrades them and improves their stats. Any weapon can have two attachments which will improve any aspect of its performance.
Powers are either tech, biotic, or weapon based. When a power is used it initiates a cool-down which prevents use of any power for a short time. Which set of powers you have access to is based on the class you have chosen. All three types create unique play styles and strategy. You can combo powers with your allies for increased effect.
As you level up in the story or multiplayer you gain skill points which can be used to either upgrade a power, increase your survivability, or increase weapon damage. At the end of the upgrades for an option it will fork into two unique upgrades that can personalize the power / play-style further.
The story mode is lengthy if you take your time and get some of the side-quests done. As with the other games, you are centralized around the ship and use it to travel between planets, explore, and interact with your crew. The loading times between decks can get annoying and important functions in the ship are often spread out. When on a planet (or in a station in some cases) you will usually get a choice of which team members to bring with you, they have their own equipment and powers that can be customized to fit your needs (you can progress their characters just as you can your own). if you import a save from the previous games, your past decisions will make minor changes in the story and some major changes in side-quests and history. During cut-scenes and dialogue choices, you have the option of leaning towards paragon (good), or renegade (bad), quick actions during cut-scenes will change how they end up and dialogue choices usually have a greater impact. The story overall is awesome, a lot of people complained about the original ending, which was expanded on by Bioware in a free piece of DLC. Story load times are short even on a traditional hard drive. Texture pop-in was not an issue.
The game looks great but is not a massive upgrade from the second game nor a breakthrough in graphics technology. The environments, character models and weapon models all look futuristic and of high-quality. Sound quality is good too, but nothing special. Though, I have to note that the game does lack in sound options.
Multiplayer is new to the series. It is a four person co-op mode where you fight through waves of enemies in a closed location. There are four difficulty levels on a variety of small-ish maps. You can choose what type of enemy you face as well; Cerberus, Reaper, Geth, or Protean. Players choose from various classes which progress similarly to the story mode but weapons, attachments, and one-time-use items/upgrades are acquired through randomized upgrade packs. Items acquired through packs have a rarity associated with them, permanent items upgrade as you get them more than once. For each class there are different race/gender combinations with different sets of powers, anything past the base human variant is acquired from packs. If you unlock the same character variant more than once more customization options become available. As you progress through the waves you will get specific missions to perform throughout the environment the faster you get through them, the more credits you get to purchase these packs. XP to upgrade your character is based on individual and team performance. Multiplayer load times can get long if you are using a traditional hard drive. Bioware has released free DLC to expand this experience to what it is today.