Not enough info for a guide here yet, but definitely enough now for a post.
I wrote Jame just before WotLK. Some friends finally convinced me to play WoW this year. I consider myself lucky I found wow-pro.com quickly. Despite much PvP and profession distraction, I reached level 70 while starting the BEM section of the guides here.
I stopped most of my questing right then so I could resume them for experience when WotLK came out.
Here’re my observations:
1) The BC quests give about 9-12k exp. WotLK quests give about 20k exp. Around half the exp, but also faster kills and about half the crowd.
2) The WotLK gear is definitely better. Just as I did while questing Classic to BC (around level 60), I suggest simply buying the advanced WotLK greens off AH and waiting to go there until at _least_ 70. Worked for my main from Classic to BC and definitely looks to work the same for BC to WotLK.
3) Ironically for those that like PvP, I suggest getting access to Dalaran and more importantly the PvP area, Wintergrasp. At level 70 I was getting over 6k exp for _each_ Wintergrasp guard during the battles there. Help one of your faction’s NPCs kill one or two of the opposing faction’s level 76 NPCs until you gain the temporary Corporal rank. From there you can build a catapult to drive around and kill them on your own. I left some battles with easily over 100k exp.
At level 74 I’m taking a break from Wintergrasp. (I’m capped on the honor.) Netherstorm questing is still giving me 9-12k exp at dark green. If people want more I’ll keep you all informed.