There's this mage on my realm who's always at the mailbox around the corner from the main Auction House in Stormwind right about the time I play another of my alts. I'm not talking about any of the mailboxes right there in the little square -- I'm talking about the one through the archway by the canal, toward the fishing quest area. Yeah, kinda out of the way, right? It's always at odd hours, when not many people are around. I can't help but notice her when she's there.
For a long time, I assumed that Mailbox Mage was just another banker alt, but then I noticed that she's actually leveling at a glacial pace. Now, as someone currently bringing up several alts simultaneously to try to sample all the revamped leveling storylines and zones, I can attest in no uncertain terms that sneezing too many times in a row nowadays will cause you to level. Mailbox Mage, though, plays pretty regularly. From what I can see, she logs in for a couple of hours at a time several times a week. She comes and goes, always heading off in different directions; she's obviously busy doing something. So I took a peek at her character profile -- instance runs and activity galore! I've come to the conclusion that that she's periodically freezing her XP, gobbling up every last bit of content in each level range before moving on.