There are two reasons I plan to stay on for WotLK.

One, I am thinking that Blizzard will apply what it learned in tBC, regarding progression and rewards, and that it will be smoother in WotLK.

Two, the intention to allow 10-mans to have their own progression separate of 25-mans. The guild I'm in is relatively small and very casual, and we don't have problems scheduling and running 10 man raids, but 25 mans have been a problem, and we don't really want to force anyone into a raiding schedule if they do not want to or cannot. So 10 mans are perfect for us, and being able to follow a progression path via 10 man instances would just be great.

And of course, enough of my friends will stay on to make it worthwhile in any event.

As for DKs, I plan to wait a while before I start one (or five). As someone mentioned, they will be the FotM, and since I'm on a PvP server they will also be the TotM (Target of the Month). The starting areas will be cramped and bloody and any areas where death knights are trying to level will be a gankfest. No thanks! I will level my mains to 80 while also working on my alts, and will consider a DK once things settle down and if I feel like doing it at that point. Our guild leader, who plays a warrior and has stuck with her main forever, will probably start a DK early and I will help her level it up when I can. If anyone deserves to try out something new, it's her!