My first two trips into the mid-30's in Runes of Magic took a year and half a dozen zones. My third trip has taken 4 nights in a single zone, the new Coast of Opportunity.
As part of adding a third class to the game, Runewaker had to offer some way for characters who had already completed the low level content while leveling their first two classes to get their new class off the ground. Adding half again the current leveling content in the game wasn't really a viable option.
The Coast of Opportunity was an interesting compromise - it's a zone where each individual quest is about as hard as a level appropriate quest elsewhere in the game, but with dramatically increased exp and gear rewards. Where a player might get a level and a mildly usable piece of green-quality gear from a series of quests in the original content, a single foray into the new area might award an entire level and one or more pieces of blue gear. Repeatable quests offer the training certificates needed to unlock your elite dual class skills far more easily than farming the old turn-ins (which are less reasonable now that each character can have up to six sets of elite skills).
The system isn't entirely for the faint of heart - I'm earning skill Training Points faster than I can figure out what to spend them on, unlocking new skills left and right, and juggling my two secondary jobs on my fledgling Warden. That said, the system works - I went in with two usable higher level classes, and I will emerge with three. The old content remains in the game for players who want to experience it, with no need for massive nerfs to difficulty or exp curves.
Overall, it's an interesting approach compared to the more traditional leveling content revamp/nerf as games age. Instead of spending cost-prohibitive amounts of time actually removing old content from the game, the developer has simply allowed those who want to bypass it to do so. It's still more elegant than outright starting characters at higher levels (e.g. WoW's Death Knights), and it doesn't really harm the previously existing experience for people who actually enjoyed it, the way that WoW's Cataclysm has.
I wonder if the hypothetical Pandaren starting area that may or may not be announced at Blizzcon will take this approach?