I should be more specific: why is this hard for me? Because for lots of people, leveling a hunter is the easiest thing in the world, to hear them tell it. It's a super easy class that has high DPS and a tanking pet to keep it alive. I should be blowing through the levels.
Instead, I've just recently managed to get my draenei hunter, who I created back when TBC launched, to level twenty eight. In the time since I've created this character I leveled my original warrior to 70, created a new draenei warrior and leveled him to 70, and leveled a draenei shaman to 70, plus went back and leveled two horde (warrior and shaman) to 70 and created a BE paladin and got him to 65. I leveled a paladin to 65 faster than I could level a hunter to 28! What is this, bizarro world? My orc and night elf warriors both hit 70 and I created them more or less as alts to play with RL friends on different servers, which means they get played once a month at most, and yet, here is my hunter, a year into the expansion (over a year, in fact) and I am just now heading into the 30's. And to be honest the only reason I even cared enough to level him this much was because I wanted to get him a mount.
I guess part of it might be a fundamental disconnect with the ethos of the class: my warlock is in even poorer shape, I literally cannot force myself to log him on. It's physically painful for me to play a class that consists of sending in the pet and doing damage from behind it. I find myself standing and meleeing on my hunter, to the point where my wife has threatened to take him away from me if I don't start learning how to kite. Since she's one of our raiding hunters I think it's also painful for her to watch me play the class so poorly.
I've recently started at least dropping traps when I see a mob break away from my pet and charge me, and I've been practicing my jump shot and how to use concussive shot to keep mobs from running up to say hello, but half the time I still forget and start swinging my two hander as soon as they get in range. It might be that 90% of my other characters are melee oriented, or it might be that I have some kind of odd brain damage that doesn't let me remember what 'ranged DPS' means, I don't know, but I find leveling the hunter to be just slightly less painful than eating a broken beer bottle burrito. I spend more time gathering ore and learning new cooking recipes because I find the actual gameplay so antithetical to my nature.
So we move on to the topic of discussion: what class did you expect to easily master and didn't? Or are you a jack of all trades and can effortlessly move from class to class, even ones with wildly varying differences in playstyle? And if so, how? How?