1): The return of my favorite characters, namely Garrus and Tali. They haven't changed a bit, and I wouldn't want them to. They are literally Shepards best bro, and the little sister he never had, provided he or she doesn't romance one of them.
Joker and EDI are also great to have back, since they make a wonderful double-act. Oh, and Wrex and Mordin. And Thane, who got much cooler this time around. And Kirrahe! And lets not forget about Grunt, and Legion. Actually, hell, lets say most of the characters from both Mass Effect 1-2, its great to see pretty much all of them again.
2): The arrival of some great NEW characters to interact with, namely Eve, Cortez, and yes, Vega. At first, I thought I'd hate him. But, I grew to like him. Hes not Garrus-level awesome, not even close. But, I definitely like him better than Jacob, who I was lukewarm about at best.
Eve, on the other hand, was an excellent look into the Krogan from a different perspective. It was touching, and added to the emotional weight behind curing the Genophage.
And lastly, Cortez, who also forms a great double-act with Vega. I wouldn't romance the guy, but hey, hes a good friend.
3): The resolution to most of the things Shepard has involved himself with over the course of the first two games, namely the Genophage and the war between the Geth and Quarians.
Genophage? I CURED THAT *****. Geth and Quarians? You two best wear some pretty friendship bracelets, or Shepard is gonna pistol-whip some fools.
... Seriously though, it was fun making peace and curing the Genophage, and it made me feel like I was not only doing the right thing in general, but doing right by my friends, Wrex, Eve, Legion and Tali.
4): The combat. Other than minor complaints about controls (I play the 360 version, and I have to say, the A button is used for a little too much. The problem is, I wouldn't know what button would be better suited.), the combat feels smooth, tight, and fun. And that brings us into 5...
5): The guns. My babies, the Mattock, Phalanx, and the Geth Plasma Shotgun are back, and they are now VANILLA, which is awesome! I loved those guns so much. And now, they've come out with more bringers-of-death than before. The Arc Pistol? The Talon? The Hornet? The Black Widow and Javelin? Oh, come to daddy.
And lets not forget the new Firefight guns, the Harrier, Geth Plasma SMG, and the Reegar. So much yes.
6): The dialogue, and the humor. Some examples:
"This is now my favorite spot on the Citadel!", "Why does the Alliance hire pilots with brittle-bone disease?", "Don't worry Shepard, I don't forget to recycle the Normandy's oxygen unless I've discovered something truly interesting.", and the awesome lines and scenes go on.
The Citadel Date with Garrus. Drunk Tali. Talking to Wrex for the first time in the War Room. Mordin singing Krogan Queen. Everything with Joker.
7): The heart-wrenching scenes. Thane dying made me cry, and I'm not ashamed to admit it, because that shit was Hughes-death levels of sad. And that was following Mordins death, which was just as hard for similar reasons. And then Legion. Goddamn Cerberus and the fucking Reapers, WHY DO YOU HAVE TO KEEP KILLING MY SQUADMATES!!
8): Going off of 7, this one deserves a number of its own: Killing Kai fucking Leng. Stay down, *****, you want Shepard to be done with you after you killed Thane. Feel lucky I didn't kill you in a more cruel and unusual fashion.