One of the early lore/codex entries tells you where Harbinger is - *very minor spoiler* - its the main Reaper assaulting earth. So its there, just not thrust in your face as much.
I actually think ME3 is probably the best of the series for the most part (not so much the end, I really think there will be a DLC expansion of the end). The combat is especially more refined. A lot more flanking, more emphasis on movement, elevations, different tactics for different units. Less find one cover and pop up shoot pop up, etc. Also the weapon/ modding system I think was really done properly. Lots of choice, lots of mods, but easy to manage. They got it right IMO. Scanning/ exploration was okay. I missed the vehicle scenes in earlier games, but not too badly.
Plot . .well you might find it corny and cliche, or you might find it moving. It does suffer from trying to tie together a lot of the plot points from the previous games, and resolve characters plots rather than focus on its own unique story all the time . .but I think it was well done. To me, I think ME3 had some of the humorous moments and some of the more poignant ones. So, since it made me have a much stronger emotional reaction than most games, I have to give it props.
The multiplayer is a bit buggy, but quite playable most of the time. Very focused on weapon/ skill unlocks. You can get any of the full endings without doing it, so its not critical (there is one small scene that takes a very high warscore that most player will need a few rounds of multiplayer to achieve). Seems like they're trying to do community events and the like to keep it lively.
Setting it up was a chore, origin is a bit of a pain, and there are some bugged quests/ dialogues (especially towards the end I had invisible characters in convos due to bugged camera angles, or people looking the wrong way). Other than the setup, I felt it was pretty forgivable for something so complex, and I think a few patches should clear up the little issues.
So won't be everyone's cup of tea, especially if you hate ME2 style combat, or are so deeply involved with ME universe lore that the deviations will enrage you. But to me it was a very good game, one that was definitely worth my time, and I'm sure I'll play again.