OK. Full spoiler review from a hardcore fan...
The Amazing - Mass Effect 3 has by far and away the best interpersonal moments of the series. There were certain sequences that made me feel legitimately nostalgic and sad, like when you go shooting with Garrus on the Citadel. Speaking with your allies and squad mates on Earth before launching the final attack was also incredible. I could go on for days about all the moments like this.
Up until the ending, the game's plot is awesome. The Mordin story, in particular, is probably the best executed and written "arc" of the series. It's absolutely perfect. As a matter of fact, the entire genophage section is pretty much perfect. I also really like how Thane's "arc" played out.
The Good - ME3 finds the perfect balance between the ridiculously complicated item inventory system of ME1 and the nonexistent item inventory system of ME2. It also finds the perfect balance between the ridiculously detailed leveling up system of ME1 and the oversimplified leveling up system of ME2.
I really enjoyed the combat in the game, although sometimes the level design is a little clumsy.
The Nitpick-y - There were a lot of tiny changes made to ME3 from ME2 that I did not like as much. I preferred the menu screens in ME2, preferred having the codex separate from the journal, etc. Just lots of nitpick-y things like that that, at the end of the day, I really don't care at all about.
The Bad - This might also be a nitpick-y thing, but I didn't like how most of the dialog options were reduced to two. I also thought there was a little too much automatic dialog.
The Horrible - I hate to be a part of the whiny internet community that is dominating discussion of ME3, but I gotta admit, I really hated the ending. I try to make sure I approach all art and entertainment without expectations or hopes, but in the case of the ME series where I ended up investing 60+ hours in a single save file and developing real relationships with the characters, I couldn't help but develop some hope that I would see what happens to them. I also wanted to see what happens to Shepard. Basically, I wanted a happy ending with lots of closure that would make me feel warm and fuzzy inside.
The ending that we actually got was just so unsatisfying. It has left me with no desire to replay ME3 anytime soon, whereas the second I finished both ME1 and ME2 for the first time I was starting a new game. I also feel like the volume of what is required of the player to achieve the best ending is so large that it makes me not want to even try. I completed 80% of the side quests in the game and still my Effective Military Strength was only at 2800. I don't mind playing multiplayer to boost that but right now I'm trying to save money so I don't have Xbox Live Gold.
Am I sounding a little self-entitled right now? Probably, and I apologize for it. I just feel that the ending sucked. It was really really bad, and makes the entirety of ME3 seem somewhat pointless.
Conclusion - Up until the ending, the game is almost perfect and has many perfect moments. The ending, however, left such a bad taste in my mouth. It made me feel sad that I will never know on any level what will happen to these characters I have come to love, and it made me feel discouraged to play the game again because it made what I'd done seem pointless.