I am disappointed.
1. Small side quests are picked up while walking around and hearing somebody talk about some item they need. Guess what, you go find that item and bring it to them. That little bit of sidequesting you actually get is pathetic. I've done only one so far, but it came down to shooting a bunch of guys in a few rooms, stretching out maybe 100-200 m
2. :/
2. The game is almost completely linear. You barely even have an illusion of any kind of open-endedness.
3. Almost no exploration of planets. At least in ME2 you could go check most planets for resources. Now you spend maybe a minute in a solar system using some scanner that attracts reapers if you use it too much and then they chase you off. Still better than nothing. At least they made the annoying probing minigame bearable that way as it goes down quickly and you don't have to spend 5 minutes each time probing a planet.
4. Dialogues are disjointed. When people talk, often it seems that they are not in a dialogue but each one of them is in a monologue, especially when trying to get to know your crew members.
I apologized to Wrex for destroying the possibly genophage cure from that salarian doctor, but there was no emotion from Shepard. It was just a robotic apology. It didn't seem to mean anything.
5. Dialogue choices are rare. In a conversation, maybe you have one of two times pop something up to gain some paragon/renegade points. It seems more like a movie than a game. Massive Fail. Mass Fail.
6. The whole time you're supposed to do something constructive against the reaper threat, but all the quests seem forced since they have almost nothing to do with the main quest. It's like "We have to do XYZ to save the galaxy. But NO! That guy won't help us until we do him some stupid favor first! Yes, the galaxy is at war and the more time we waste, the reapers kill more millions of people. But NO! It's ok! Because this guy forgot his fucking pants on that moon and he won't help us until we get them." :/
But not all is bad. They at least made the skills simpler to upgrade and added some choices. Weapons can be upgraded simpler as well and now you have "weight" which determines how many weapons you can carry around. If you go over that weight (which you actually can do), it increases the time required for your skills to recharge. And you don't have to pay attention to every single bullet you're firing so you don't run out of ammo like in ME2. But apart from that no actual improvements. Battle is still the same. It was good, but one would expect some additions, some changes, something interesting and new. :/
What I mentioned under #6 shows how stupid politicians are. The reapers are tearing apart the galaxy and all their care about is yet again getting something they want to gain some sort of advantage (e.g. Wrex which I was very disappointed in).
All in all: weak. I did expect that they would screw up some things with ME3 like they did with ME2, but in all honesty, I didn't expect this. If ME1 was 100, ME2 was 85, then ME3 is like 40. They stripped away all the good stuff and left us with some casual crap hybrid that's barely enjoyable.
I'd even go as far to compare it to Duke Nukem Forever. It was ok, but meh. ME3 is like that. It's ok (especially if you want to see how the story plays out), but it's meh. Even the characters lost their edge. I loved in ME2 how Garrus became much more badass than he was in ME1, but in ME3 he's meh, just no edge. :/
I recommend it to everybody who's played ME1 and ME2 and has a save from ME2 (possibly imported from ME1 as well). But for the rest, don't play it. You won't enjoy it.
EDIT: While talking with a colleague from work at lunch, he mentioned that the main writer left after ME2 and they hired some Hollywood jerk instead. It shows. :/