ok,some impressions(few hours in,chapter 4)
-no backtracking,you just keep moving forward
-in addition to old enemies,few new ones
-story so far is kinda non-existent(you have to come to some place,but as usual something is always in your way)
-graphically they improved some things(shadows)
-loading times are quick,checkpoints are good
-hardest(that you can choose) difficulty is ok,not something impossible(frustrating)
-you have to chose the right strategy against right enemy,when attacked in groups you can use that to your advantage
That said,game has that Bioshock 2 feel,at least to me.It's far more improved than Bioshock 2,but it still carries that familiar feel .Sometimes game gives me Bioshock vibe,when you play it you will understand.
It's a great game,very polished and fun to play,but if you are expecting something different,don't...it is certainly not like ME1 to ME2 transition
I'm only like few hours in,so I could be too harsh(in that familiar thing)
One more thing,you can backtrack if you want,unless some major disaster is in your way...game won't stop you otherwise