As far as gameplay goes, it's been improved over Mass Effect 2 a lot. The improved skill trees are awesome, I love heavy melee (especially for the Adept; it owns husks and even Cerberus troopers. Each class has their own and the Soldier has the worst one; barely any range or width on it at all
) and the ability to grab enemies from over thin cover and instakill them, the new weapons and weapon modifications are cool. (Like Darkmere said, AP ammo + Armor Piercing Mod + Revenant makes short work of any enemy, in cover or not.) I haven't gotten the hang of transitioning (there's some way you're supposed to tap A when running at cover to slide over it; I keep sliding into it instead) yet though.
Multiplayer is a blast if you get a team actually interested in teamwork rather than killcounts; I've had games where if I had lived we would have won (not to be a douchebag, but let's say we're on Wave 10 and the objective is find four objects and activate them. Generally I'll get two or three of them and then get killed while trying to get the fourth because no one's assisting me, and then I just watch everyone else run around like dumbasses killing enemies until they run out of time and fail the game) but I've also had games where my teammates were completely awesome. I find myself wishing the previous two Mass Effects had this gameplay.
That aside, I liked many of the missions. The environments were more varied, and the inclusion of ladders really improved the feel of each area (to me, anyway). The atmosphere was great, too. Missions in which you were dropped into a warzone felt much more frantic and realistic than other sci-fi games of a similar sort (I mean, I love Gears, but it never felt as much like an actual war as ME3 does). I like the three new party members (one I'm not going to spoil although if you've played beyond the first four missions you already have her, but still); the DLC teammate is decent although for some inexplicable reason he sounds just like Mr. Sunshine from Saint's Row 2 to me, and James is a lot less bland than Kaidan or Jacob. I like Jack's new design, although both Miranda and Jacob staying in their old uniforms is stupid considering their roles in this game.
Especially Miranda. "lol I'm hiding from Cerberus by continuing to wear the same white skintight outfit they assigned me btw let me walk into frame with the camera positioned at ass level." Don't get me wrong, she has a great ass, but lern2subtlety, Bioware, kthnxbai.
My first time through was with a Liara-romancing Adept and an assault rifle-class weapon the whole time; this time is a Kaidan-romancing Soldier with sniper rifles and I have to say I love the Viper in this game even though it sucked in Mass Effect 2. I'll probably end up playing ME3 enough to see all the different angles (still have Ashley, Miranda, Tali, Jack, Garrus, Thane, and Jacob to go through after this run, and then I'll have to build a chaste Shep for the first two games so I can try out the options in this game), even though I do have some ending-rage like others in the thread. I guess I can forgive it because the plot is awesome (up to that point >.>) and there were some actual thought-provoking decisions from me at times.