I could just ask 'what I need to know' but vague questions are just going to get me more vague answers, so here's a few things that I find to be mildly pressing. On top of that, a lot of the information I can find (both regarding LW and EW) is painfully vague.
I knew Long War was going to be more difficult going in, but I wasn't nearly expecting it to be -this- hard. So I just recently beat Enemy Unknown (on normal) and, wanting a more difficult game when I went in to Enemy Within, installed Long War.