I found following the guide in BC, the only really long quest was the zepellin crash site, which wanted 30 pieces off of the ground per toon.

Conversely, in WotLK, there are so many more collections. I don't think you could level from 70 to 80 on kill quests alone. You'd have long stretches where you'd need to grind quite a bit before reaching the minimum level to take the next set of quests. I suppose rested experience can go a long ways towards helping this.

If you were to instance for a 10 level segment and then quest for a 10 level segment, I'd definitely quest in Burning Crusade and instance in Wrath. Plus that way, if you ever feel like doing the Wrath quests, they're 13-22g each; BC quests at 80 are in the 3g range, at least the Shadow Moon Valley ones are, and that's the highest BC zone.