IMO, the Cata levels were too small on release, I think it took me a couple of days to get to 85, and I know people who did it in 20 straight hours of playing, cause they are completely crazy. That might just be because there were only 5 levels.
I explained it before but the issues with the levels was they attempted to pack to much into only five levels. Yes, the levels
as a whole took less time then the past times we had ten levels, but each level itself felt progressively longer. Here is my chart I made to represent what I mean.
WOTLK 70-80.
70--71--72--73--74--75--76--77--78--79--80
Cata 80-85
80--81---82----83------84---------85
Milestones are extremely important, and a smooth line of levelups reduces the risk of burnout, because that next level (milestone) always feels in reach. When each level gets progressively longer, the milestone feels farther each time. This is not a huge deal during the first playthrough, but gets worst the more alts you have, because once you reach the new milestone the next one feels so far away you just don't care to play at that point.
I would much rather have 10 levels with an even line of progression then 5 with such a drastic curve, but sadly can't do much about that, since they need to drag out as close as they can to 10 levels of leveling into only the 5 levels.