For invasions (developed from reading the help of all the people posting here):
I began writing down level times last Friday, when I started leveling a horde shaman from 1 to 100.
1 - 100 took 16 hours and 7 minutes. I hit 80 at the 10h 20m mark
Yesterday, I started leveling an Alliance Hunter.
I am at 80 and my time is 8h 20m. I hit 60 at the 4h mark.
So, for the shaman, I wanted some boxes to open at 100. I traveled around much more; at about 60 I would do the first 2 invasions in the set of 3, then sit in the last one, doing phase 1 to 3, then relogging. I did the last invasion until the zones rotated. Last week also had the invasions moving around, so there was a chance every 2 hours that it forced you to move. When I hit 85, I realized that gathering boxes could have waited until Cata portals opened, and it would have cut down on travel time.
For the Hunter, I went to invasions at level 10 (same as shaman), and from 10 to 60 I did phase 1 and 2, then relogged to reset. At level 60 I went to Stormwind and got flying. Went back to Westfall and started doing phases 1, 2 and 3. In phase 3, for Westfall, you have 3 skull bosses in the south, that the horde will kill (one can be bugged out). Once all 3 of those are killed, two are due to spawn in Westfall. Run up and kill them, and immediately relog. I would also recommend getting a stack of 5 bandages from the illidari camp, north of Westfall, unless you have very good self heal. Even with the Hunter 70% self heal, on 2 min cooldown, I go through 5 bandages in about 5-6 invasions.
How I would rate XP from various invasions:
Westfall
Xroads (N Barrens)
Azshara
Tanaris
Hillsbrad
Dun Morogh (I hate this invasion)
Westfall and Xroads are close. I think Westfall would be a clear winner, if it wasn't for the bugged boss in phase 3.
Where you killing Abysal in the Westfall invasion? I know one of the treasure goblins will turn you into a skelly. Some other npcs may turn you into one in other invasions as well.