OK, my bad, this is a famous article, which I assumed more people had previous knowledge of.
It boils down to a little equation:
(Time needed to level) - (Time needed to burn content) = Work
You want to minimize the "Work" variable, so either make levelling faster, or make content less "burnable"... This could mean adding more 50-60 zones, and even more zones for AA grinding (because you'll already have burned zones on the 50-60 climb). Or it could mean re-doing levels / content in some zones dynamically. Or it could mean making the exp faster. Or altering some of the game dynamics - exp for tradeskills? exp for duels? meaningful exp for quests? Or (as thott suggested) making the exp in a particular dungeon only good for a very small level range, like 1. This wouldn't work on WR, cause you usually have like one or two people at any given level...
Right now, for the very steep requirements of 50-60, we have: half of SG, half of DN, half of the Hole, half of Seb, half of Kael, ToFS?, half of ... erm thats all i can think of. 3.5 zones worth, in total... And considering that all the geometry for these zones is well known to all (WR's special take on the Kunark problem - no new zones are really new), you have maybe 50 hours on those 3.5 zones before content burn.
In those 50 hours, you might get to 53rd level.
This doesn't even start counting AA-grind time... You might have some more options available to you in a full group of 60s - say ToT and a couple more places around town - but its going to get old real fast.