My levelling has slowed down lately, and it's not just because fifty to sixty takes a while. There's a good reason and a bad reason.
Let's get the bad out of the way first. I just don't really like most of the Vanilla level 50+ zones. The Plaguelands are depressing, the Burning Steppes are dull, Silithus with its grating soundtrack and ongoing bug noises is just freaking annoying. I actually find myself wondering if WoW had been as "sticky" for me if Burning Crusade hadn't come out relatively soon after I hit max level, sparing me from having to spend a significant amount of time with the level sixty endgame. If nothing else, I probably would have focused even more on rolling alts. This means that right now, I do a lot of flitting back and forth between zones and get tired of them really quickly. A couple of quests in Searing Gorge, then one in Azshara, then a few in Felwood, but nowhere really sticks (though I haven't done Un'Goro yet, at least that should be fun). It's not that there's a lack of content to level up, I'm just not keen enough on a lot of it.
On the plus side, part of why my levelling has slowed down is that I'm finding myself thinking a bit more about things other than max level. It may be slow, but I'm getting there, and while I have no plans to raid, there are still a lot of goals to achieve. First off, there are professions. My ongoing battle with armoursmithing will deserve a post of its own, but let's just say right now that levelling certain professions requires a serious investment of time and effort. I'll also need lots of money to get my epic mount, and of course pallies have their own special quest for that. So I'm finding that I'm less focused on gaining maximum XP during a play session and instead tend to get distracted by other concerns, such as making money or levelling fishing for a bit.
I even tried a bit of PvP. I saw people campaigning very heavily in the past for everyone of level 51 or above to queue up for Alterac Valley on AV weekend, so when it came around again and I was finally of a high enough level to join in, I gave it a go. I only played three matches, all of which were losses, and I felt pretty damn useless at my level... but it was still kind of fun simply because it was so chaotic, with all those actually powerful NPCs killing people left and right. I just tried to throw out a few heals before dying. I did get a couple of the PvP quests done in the process, and I had fun watching a hunter slowly kite Galvangar to death.
But that's part of the beauty of Vanilla, that it's so multi-faceted and that getting to max level is not the only thing on your mind.