Here's a summary:
-15-20 hour-long main quest
-Around 40 hours playtime in total- side missions are vital to amass enough opposition to the Reaper threat
-90 minutes of cutscenes
-No loose ends or cliffhangers, the game is truly an appropriate end to the trilogy with a very intense and touching finale
-More than 100 scenes, conversations and/or events change depending on our choices after importing a save-game from ME2- from something small like someone thanking us for completing a mission to bigger repercussions which would be to spoilerous to write down- you are definitely going to feel the consequences, and not always like them
-SPOILER: In order to rally allies, we search for a cure for the Genophage, build a Prothean weapon, disarm a Cerberus bomb before it destroys half of a moon, and are even inserted virtually in a computer program
-If and how many of these quests we complete influences how the game ends, and who- individually and globally- survives
-While the game can be enjoyable for newcomers as well, because the game is strong on its own mechanically and story-wise, it is recommended to play the first two entries to make more sense out of the wealth of information that the game builds on
-There's an interesting sequence after the end credits
-The „Role-Play“ mode of the game (In contrast to „Action“ and „Story“) feels very much like ME1
-The game gives you more opportunity to customize your Shepard. Each class has 8 talents with 6 levels each, and up from Level 4 there are two branches to choose from to level up each individual talent differently. Weapon customization is also a factor and noticeable, but the effects (which have both pros and cons) are only marginal
-Balancing isn't optimal
-The game is noticeably more difficult up from „Medium“, which makes using the correct weapon mods and talents integral to survive. Shepard also shoots more accurately in cover.
-The AI, on the other hand, is still not very advanced
-SPOILER: You are going to visit Krogan ruins, a Geth ship and the Turian moon Palaven, among others
-There are multiple turret sections which aren't very well directed
-Despite the shortcomings in level design, which are similar to ME2, the game is constantly thrilling, spectacular and dramatic- largely thanks to the well-presented story and dialogue- and there are lot of small and large choices again to ponder on
-Scanning isn't nearly as repetitive as before, because it works a lot quicker now and incorporates a new mini-game where you have to navigate your ship away from Reaper threat
-The new melee option is fun, but a little overpowered
-Graphically, the low polycount, lack of detail and low texture resolution disappoints- the characters are well animated and modeled however, and the overall look, despite its age, is quite moody
-Great dynamic soundtrack and sound-effects
-Our progress in the war effort is constantly trackable on a terminal on the Normandy
In summary, the game builds on the strenghts of ME2 and takes some notes from ME1 to create the most compelling experience in the franchise yet.