You still need to use LJ to read LJ friends, afaict. If you post on DW, you can crosspost to LJ or not. You can allow comments on LJ or not. The crossposting is pretty good, in fact, so edits you make here will propagate properly and such. I have readers on both, so for now I'm just crossposting.

I went through LJ last year and unfriended everybody who hadn't touched it in over, I think 2 years (looking for the later or when they last posted plus last time I saw a comment from them, thanks to gmail archive searching). I re-added only one in like 200 who was reading but hadn't commented or posting in that long.